The new "share sheets" that Facebook is working on will unify sharing across the primary Facebook apps, including Facebook Groups and Messenger. The idea is to bring "the best sharing experience" to all of Facebook's apps instead of having multiple experiences to deal with varying experiences.
The idea of the new sharing sheet is to make things easier for users so they know what to expect no matter what Facebook app they're sharing to. Additionally, the new share sheet will also help you to see what your shared content is going to look like when you submit it onto Facebook.
Facebook didn't give an expectation of when we should start seeing the new share sheets available for Facebook app users, but we're hopeful it'll be coming soon.
Source: Facebook F8