You can only launch an installed app using the Custom -URL. But you cannot check whether the app is installed or not (Second part of your question.) Reason: If it would have been possible to check the installed apps in iOS, One can then create app which will get information of all installed apps.
facebook Custom URL latest updates:
fb://album/%@
fb://album/(aid)
fb://album/(aid)/cover
fb://album/(initWithAID:)
fb://album/(initWithAID:)/cover
fb://album/new
fb://albums
fb://birthdays
fb://birthdays/(initWithMonth:)/(year:)
fb://birthdays/(month)/(year)
fb://chat/(fbid)
fb://chat/(initWithUID:)
fb://chat/(user.fbid)
fb://contactimporter
fb://contactimporter/invites
fb://contactimporter/legalese
fb://contactimporter/modal
fb://event/%@
fb://event/%llu
fb://event/(event.fbid)/members/(rsvpStatus)
fb://event/(fbid)
fb://event/(fbid)/members/attending
fb://event/(fbid)/members/declined
fb://event/(fbid)/members/not_replied
fb://event/(fbid)/members/unsure
fb://event/(fbid)/rsvp
fb://event/(initWithEventId:)
fb://event/(initWithEventId:)/members/(rsvpStatus:)
fb://event/(initWithEventId:)/rsvp
fb://events
fb://events/
fb://faceweb/(initWithURL:)
fb://facewebmodal/(initWithURL:)
fb://feed
fb://feed/%@
fb://feed/(filter.filterKey)
fb://feed/(initWithFilterKey:)
fb://feedfilters
fb://findfriends
fb://findfriends/legalese
fb://findfriends/modal
fb://friends
fb://friends/picker
fb://friends/sync
fb://friends/sync/(removeData:)
fb://friends/sync/disconnect
fb://friends/sync/legalese
fb://group/(fbid)/members
fb://group/(initWithGroupId:)/members
fb://groups
fb://launcher
fb://mailbox
fb://mailbox/(folder)
fb://mailbox/(initWithFolder:)
fb://mailbox/(initWithFolder:)/(tid:)
fb://mailbox/(mailbox.folder)/(tid)
fb://mailbox/compose
fb://mailbox/compose/(fbid)
fb://mailbox/compose/(initWithUID:)
fb://map
fb://messaging
fb://messaging/(folder)
fb://messaging/(initWithFolder:)
fb://messaging/(initWithFolder:)/(tid:)/participants
fb://messaging/(initWithFolder:)/thread?tid=(tid:)
fb://messaging/(mailbox.folder)/(urlEscapedTid)/participants
fb://messaging/(mailbox.folder)/thread?tid=(urlEscapedTid)
fb://messaging/compose
fb://messaging/compose/(fbid)
fb://messaging/compose/(initWithUID:)
fb://messaging/original_message?mid=(commentId)
fb://messaging/original_message?mid=(initWithMessageId:)
fb://nearby
fb://note/%@
fb://note/(initWithNoteId:)
fb://note/(initWithNoteId:)/edit
fb://note/(noteId)
fb://note/(noteId)/edit
fb://note/compose
fb://notes
fb://notifications
fb://online
fb://online#offline
fb://online#online
fb://page/(page.id)
fb://pages
fb://photo/%@/0/%@
fb://photo/(album.user.fbid)/(album.aid)/(pid)
fb://photo/(album.user.fbid)/(album.aid)/(pid)/feedback
fb://photo/(fbid)/profilepic
fb://photo/(initWithProfilePicturesUID:)/profilepic
fb://photo/(initWithUID:)/(aid:)/(pid:)
fb://photo/(initWithUID:)/(aid:)/(pid:)/feedback
fb://photosapp
fb://place/%@
fb://place/(initWithPageId:)
fb://place/(targetId)
fb://place/addfriends
fb://place/addphoto
fb://place/create
fb://places
fb://places/%lld/%lld
fb://places/(initWithCheckinAtPlace:)/(byUser:)
fb://places/legalese/tagged/%lld/%lld
fb://places/legalese/tagged/(initWithTaggedAtPlace:)/(byUser:)
fb://post/%@
fb://post/%@_%@
fb://post/(initWithPostId:)
fb://post/(initWithPostId:)/tagged
fb://post/(postId)
fb://post/(postId)/tagged
fb://post/(postId)/untagSelf
fb://post/(untagSelfFromPostWithId:)/untagSelf
fb://profile
fb://profile/
fb://profile/%@
fb://profile/%lld
fb://profile/(addFan:)/addfan
fb://profile/(fbid)
fb://profile/(fbid)/addfan
fb://profile/(fbid)/addfriend
fb://profile/(fbid)/fanpages
fb://profile/(fbid)/fans
fb://profile/(fbid)/favorite
fb://profile/(fbid)/friends
fb://profile/(fbid)/info
fb://profile/(fbid)/menu
fb://profile/(fbid)/mutualfriends
fb://profile/(fbid)/photos
fb://profile/(fbid)/poke
fb://profile/(fbid)/removefriend
fb://profile/(fbid)/wall
fb://profile/(initWithFBID:)/menu
fb://profile/(initWithFansUID:)/fans
fb://profile/(initWithFriendsUID:)/friends
fb://profile/(initWithInfoUID:)/info
fb://profile/(initWithMutualFriendsUID:)/mutualfriends
fb://profile/(initWithPhotosUID:)/photos
fb://profile/(initWithUID:)
fb://profile/(initWithUID:)/addfriend
fb://profile/(initWithUID:)/fanpages
fb://profile/(initWithUID:)/poke
fb://profile/(initWithUID:)/removefriend
fb://profile/(initWithWallUID:)/wall
fb://profile/(toggleFavorite:)/favorite
fb://profile/(user.fbid)/fans
fb://profile/(user.fbid)/friends
fb://profile/(user.fbid)/mutualfriends
fb://profile/0
fb://publish
fb://publish/mailbox/(initWithFolder:)/(tid:)
fb://publish/mailbox/(mailbox.folder)/(tid)
fb://publish/photo/(album.user.fbid)/(album.aid)/(pid)
fb://publish/photo/(initWithUID:)/(aid:)/(pid:)
fb://publish/post/(initWithPostId:)
fb://publish/post/(postId)
fb://publish/profile/(fbid)
fb://publish/profile/(initWithUID:)
fb://publish/profile/(owner.fbid)
fb://requests
fb://root
fb://upload
fb://upload/%@/album/%lld/%@
fb://upload/%@/checkin/%lld
fb://upload/%@/profile/%lld
fb://upload/(initWithSource:)/album/(uid:)/(aid:)
fb://upload/(initWithSource:)/checkin/(checkinId:)
fb://upload/(initWithSource:)/profile/(uid:)
fb://upload/actions
fb://upload/actions/album/(initWithUID:)/(aid:)
fb://upload/actions/album/(user.fbid)/(aid)
fb://upload/actions/checkin/(checkinId)/
fb://upload/actions/checkin/(initWithCheckinId:)
fb://upload/actions/newalbum
fb://upload/actions/profile/(fbid)
fb://upload/actions/profile/(initWithUID:)
fb://upload/actions/resume
fb://upload/album/(showUploadMenuWithUID:)/(aid:)
fb://upload/album/(user.fbid)/(aid)
fb://upload/checkin/(checkinId)
fb://upload/checkin/(showUploadMenuWithCheckinID:)
fb://upload/discard
fb://upload/profile/(fbid)
fb://upload/profile/(owner.fbid)
fb://upload/profile/(showUploadMenuWithUID:)
fb://upload/resume
fb://userset
fb://video/%@
fb://video/(playVideoWithId:)
fb://video/(videoId)