I highly, strongly (!) recommend to give users the option to share posts! Every social media loves from interaction, and that means also that users can share content from other users which they think could be of interest for their followers or connected people. Of course, the poster of the original content should have the option to prevent the sharing of content which he thinks should stay within a certain private circle, but sharing should not be impossible per se. This would be idiotic (yes, I use strong language here)!