There are quite a few websites and online platforms that have been created specifically for musicians and their work, but whether they qualify as social media or not depends a bit on the user’s perspective.
When looking solely at social media sites, names like Drooble, Playlist, Mugo, Groupie, and Triller all come to mind. None of these are massively popular like the other apps mentioned in this article, but they are focused solely on music, while the others aren’t.
There are a number of other sites that are great for musicians to have a presence on, but they don’t really count as “social media,” at least not in the way most people think of that phrase. Bandcamp, ReverbNation, SoundCloud, and surely there are others, but these are the biggest and most popular.