Facebook just passed its peak. Back in 2009 it was the new thing so there was an explosion of new users, by now it's just a classic in the game, so the majority of people have and use Facebook. It's still very well established, and not really in decline. No other social media has managed to replace it so far, because no other social media does it like Facebook does. I've seen plenty of lookalikes trying and failing, and the social medias that do make it, make it because they're developing their own unique concept that wasn't occupied by anyone yet.