UIActivityViewController doesn't show FB and Twitter on iOS 7
They won't show when you don't have Twitter and Facebook accounts set up in settings. Facebook or twitter app are not enough - you should have explicitly set accounts.
Maybe it's not really cool, (user may want to share something on fb or twitter and perform login for it) but Apple considered these activities should work so in iOS 7.
As Ahmed said above There is a problem in IOS 8.3 to use Facebook text sharing even if you add Facebook account in the setting.
I recently post thisproblem in sharing facebook text using uiactivityviewcontroller
In my opinion this is a bug that apple have to fix.maybe in the next version...