if discord does count i think that'd be my favorite. otherwise i'm not sure. lots of social media platforms are really poorly maintained, in more ways than one.
Sometimes I really enjoy using Twitter since I can spew nonsense into the void while no one would care.
I'm not exactly sure what kind of platforms count as social media but I like forums a lot.
Used to be YouTube, but there's a lot of crap on there now. Right now I don't have any social media, but if I did I'd choose Facebook. It's the main source of interaction for me and my friends.
Hmm, most likely tumblr. I love the creativity people have on layouts and it's pretty nice for gif-sets or pretty images. If you follow the right kind of blogs, it's easy to lose time or can be used as a studying tool (language tumblrs).
Don't have any current favorites, unless Discord, Kik, and this forum count as social networks? But once I'm caught up with my work I'm thinking of being more active on IG, Tumblr, and Twitter.