I never really used it but the Google+ 'circles' to which to share something to was pretty cool. ie public/friends/specific groups
mastodon should have that tbh, just locked account + followers-only as an all-or-nothing sucks
@f0x Facebook having their lists of friends is also good, but they've made it harder to manage those recently.
It'd be nice if there were a "mutuals" setting.
@f0x google+ honestly had some really good concepts going