This is an excellent list of all the official Twilight-related social media.  Head over here and thank Amanda at Twilight Examiner for putting it together.

Books

Stephenie Meyer (Author, Twilight series books and The Host)

Official website – StephenieMeyer.com

Little, Brown and Company (Publishing group, Twilight series and The Host)

Official website – HatchetteBookGroup.com
The Twilight Saga - TheTwilightSaga.com

Filmmakers

Summit Entertainment (Film studio, Twilight series films)

Official websites – NewMoonTheMovie.com, EclipseTheMovie.com, Summit-Ent.com
Official Facebook Fan Page – Facebook.com/Twilight
Official MySpace page – Myspace.com/Twilight
Official Twitter page – @Twilight
Official YouTube page – Youtube.com/OfficialTwilightFilm

Melissa Rosenberg (Screenwriter, Twilight series films)

Official Facebook page – Facebook.com/Melissa.Rosenberg

David Slade (Director, The Twilight Saga: Eclipse)

Official Twitter page – @David_A_Slade

Cast

(in alphabetical order by last name)

Charlie Bewley (“Demetri”)

Official website – CharlieBewley.com
Official Twitter page – @Alchemission

Gil Birmingham (“Billy Black”)

Official website – GilBirmingham.com
Official Facebook page – Facebook.com
Official MySpace page – MySpace.com/GilBirmingham
Official Twitter page – @GilBirmingham

Jamie Campbell Bower (“Caius”)

Official Twitter page – @JamieBower

Cameron Bright (“Alec”)

Official Twitter page – @CameronBright

Billy Burke (“Chief Charlie Swan”)

Official Twitter page – @Billy_Burke

Justin Chon (“Eric Yorkie”)

Official Twitter page – @JustinChon
Official YouTube page – YouTube.com/JustinChon

Daniel Cudmore (“Felix”)

Official Facebook page – Facebook.com
Official Twitter page – @DanielCudmore

Peter Facinelli (“Dr. Carlisle Cullen”)

Official website – Peter-Facinelli.com
Official Facebook page – Facebook.com
Official MySpace page – MySpace.com/PeterFacinelli
Official Twitter page – @PeterFacinelli

Jodelle Ferland (“Bree Tanner”)

Official Twitter page – @JodelleMicah

Edi Gathegi (“Laurent”)

Official MySpace page – MySpace.com/FrameWorkEdi
Official Twitter page – @IamEdiGathegi

Kiowa Gordon (“Embry Call”)

Official MySpace page – MySpace.com/KiGordon
Official Twitter page – @CircaKiGordon

Ashley Greene (“Alice Cullen”)

Official Twitter page – @AshleyMGreene

Anna Kendrick (“Jessica Stanley”)

Official Twitter page – @AnnaKendrick47

Tinsel Korey (“Emily Young”)

Official Website – TinselKorey.com
Official charity page – 4real.com/TinselKorey
Official MySpace page – MySpace.com/TinselKorey
Official YouTube Page – @TinselKorey

Rachelle Lefevre (“Victoria” in Twilight and The Twilight Saga: New Moon)

Official charity page – Susan G. Komen
Official Twitter page – @RachelleLefevre

Alex Meraz (“Paul”)

Official MySpace page – MySpace.com/NomaK1000
Official Twitter page – @TheAlexMeraz

Christian Serratos (“Angela Weber”)

Official MySpace page – MySpace.com/ChristianSerratosMusic
Official Twitter page – @CSerratos

Michael Sheen (“Aro”)

Official Twitter page – @MichaelSheen

Chaske Spencer (“Sam Uley”)

Official website – ChaskeSpencer.com
Official Facebook page – Facebook.com
Official MySpace page – MySpace.com
Official Twitter page – @TheRealChaske

Boo Boo Stewart (“Seth Clearwater”)

Official MySpace page – MySpace.com/ActionBooBoo
Official Twitter page – @Mammarazzi1 (shared with sister and mother)

Solomon Trimble (“Sam” in Twilight)

Official MySpace page – MySpace.com/SolomonTrimble

Michael Welch (“Mike Newton”)

Official website – MichaelWelchOnline.com
Official Facebook page – Facebook.com
Official MySpace page – MySpace.com/OfficialMichaelWelch

Score Composers

Carter Burwell (composer, Twilight)

Official website – CarterBurwell.com

Alexandre Desplat (composer, New Moon)

Official website – AlexandreDesplat.net

Howard Shore (composer, Eclipse)

Official website – HowardShore.com

Soundtrack Musicians

(in alphabetical order)

Anya Marina (“Satellite Heart,” New Moon)

Official website – AnyaMarina.com
Official Facebook page – Facebook.com/AnyaMarinaMusic
Official MySpace page – MySpace.com/AnyaMarina
Official Twitter page – @AnyaMarina
Official YouTube page – YouTube.com/AnyaMarinaMusic

Band of Skulls (“Friends,” New Moon)

Official website – BandofSkulls.com
Official MySpace page – MySpace.com/BandofSkulls
Official Twitter page – @BandofSkulls
Official YouTube page – YouTube.com/BandofSkulls

Bobby Long (co-writer, “Let Me Sign,” Twilight)

Official website – MusicBobbyLong.com
Official Facebook page – Facebook.com
Official MySpace page – MySpace.com/MusicBobbyLong
Official Twitter page – @BobbyLongNews
Official YouTube page – YouTube.com/MusicBobbyLong

Black Rebel Motorcycle Club (“Done All Wrong,” New Moon)

Official website – BlackRebelMotorcycleClub.com
Official Facebook page – Facebook.com
Official MySpace page – MySpace.com/BlackRebelMotorcycleClub
Official Twitter page – @BRMCOfficial
Official YouTube page – YouTube.com

Blue Foundation (“Eyes on Fire,” Twilight)

Official website – DPC-rec.dk
Official MySpace page – MySpace.com/BlueFoundationDenmark
Official Twitter page – @BFHome

Bon Iver (co-performer, “Roslyn,” New Moon)

Official website – BonIver.org
Official MySpace page – MySpace.com/BonIver

Collective Soul (“Tremble for my Beloved,” Twilight)

Official website – CollectiveSoul.com
Official Facebook page – Facebook.com/CollectiveSoul
Official MySpace page – MySpace.com/CollectiveSoul
Official Twitter page – @Collective_Soul
Official YouTube page – YouTube.com/CollectiveSoulTV

Death Cab for Cutie (“Meet Me on the Equinox,” New Moon)

Official website – DeathCabforCutie.com
Official Facebook page – Facebook.com/DeathCabforCutie
Official MySpace page – MySpace.com/DeathCabforCutie
Official Twitter page – @DCFCTours
Official YouTube page – YouTube.com/DCFCTV

Editors (“No Sound but the Wind,” New Moon)

Official website – EditorsOfficial.com
Official Facebook page – Facebook.com/EditorsMusic
Official MySpace page – MySpace.com/EditorsMusic
Official YouTube page – YouTube.com/EditorsOfficial

Grizzly Bear (“Slow Life, New Moon)

Official website – Grizzly-Bear.net
Official MySpace page – MySpace.com/GrizzlyBear

Hurricane Bells (“Monsters,” New Moon)

Official Facebook page – Facebook.com/HurricaneBells
Official MySpace page – MySpace.com/HurricaneBells
Official Twitter page – @HurricaneBells

Iron & Wine (“Flightless Bird,” Twilight)

Official website – IronAndWine.com
Official Facebook page – Facebook.com
Official MySpace page – MySpace.com/IronAndWine

Linkin Park (“Leave Out All the Rest,” Twilight)

Official website – LinkinPark.com
Official Facebook page – Facebook.com/LinkinPark
Official MySpace page – MySpace.com/LinkinPark
Official Twitter page – @LPUnderground
Official YouTube page – YouTube.com/LinkinParkTV

Lykke Li (“Possibility,” New Moon)

Official website – LykkeLi.com
Official Facebook page – Facebook.com/LykkeLiOfficial
Official MySpace page – MySpace.com/LykkeLi
Official Twitter page – @Lykke_Li
Official YouTube page – YouTube.com/LykkeLiVideos

Marcus Foster (co-writer, “Let Me Sign,” Twilight)

Official MySpace page – MySpace.com/MarcusFoster
Official Twitter page – @MarcusFoster1

Muse (“Supermassive Black Hole,” Twilight; “I Belong to You,” New Moon)

Official website – Muse.mu
Official Facebook page – Facebook.com/Muse
Official MySpace page – MySpace.com/Muse
Official Twitter page – @Muse
Official YouTube page – YouTube.com/Muse

MUTEMATH (“Spotlight,” Twilight)

Official website – MUTEMATH.com
Official Facebook page – Facebook.com/MUTEMATH
Official MySpace page – MySpace.com/MUTEMATH
Official Twitter page – @MUTEMATH
Official YouTube page – YouTube.com/MUTEMATH

OK Go (“Shooting the Moon,” New Moon)

Official website – OKGo.net
Official Facebook page – Facebook.com/OKGo
Official MySpace page – MySpace.com/OKGo
Official Twitter page – @OKGo
Official YouTube page – YouTube.com/OKGo

Paramore (“I Caught Myself” and “Decode,” Twilight)

Official website – Paramore.net
Official Facebook page – Facebook.com/Paramore
Official MySpace page – MySpace.com/Paramore
Official Twitter pages – Source
Official YouTube page – YouTube.com/Paramore

Perry Farrell (“Go All the Way (Into the Twilight),” Twilight)

Official website – PerryFarrell.com
Official Facebook page – Facebook.com
Official MySpace page – MySpace.com/PerryFarrell
Official Twitter page – @PerryFarrell

Sam Bradley (co-writer, “Never Think (Too Far Gone),” Twilight)

Official website – SamBradley.com
Official MySpace page – MySpace.com/SamBradley
Official Twitter page – @SamuelTwitt1

Sea Wolf (“The Violet Hour,” New Moon)

Official website – SeaWolfMusic.com
Official Facebook page – Facebook.com
Official MySpace page – MySpace.com/SeaWolf
Official Twitter page – @SeaWolfMusic

St. Vincent (co-performer, “Roslyn,” New Moon)

Official Twitter page – @St_Vincent

The Black Ghosts (“Full Moon,” Twilight)

Official MySpace page – MySpace.com/BlackGhosts

The Killers (“A White Demon Love Song,” New Moon)

Official website – TheKillersMusic.com
Official MySpace page – MySpace.com/TheKillers
Official Twitter page – @TheKillers

Thom Yorke (“Hearing Damage,” New Moon)

Official MySpace page – MySpace.com/ThomYorkeMusic