Are there any plans to reintroduce Twitter and Facebook sharing services in the project targets pane?
Yes, in the next major (paid) update. This isn’t a ruse to make you pay extra, though. The problem is that as of 10.10, Apple updated the ShareKit framework in such a way that it will no longer work with 32-bit apps - so Apple essentially made it so that we could no longer use these features in Scrivener 2.x for users on Yosemite and above. The next major update will be 64-bit, though, so will be able to reintroduce these features. The next major update will be available sometime in 2016, although it’s too early right now to say whether that will be early or late 2016, I’m afraid.