Ello is a personal favorite of mine. It’s a social media network for creatives, just as you say. I’m seeing people actively go dark on Facebook and more and more people are leaving Twitter for Snapchat. Tumblr is dying/has died/has been dying for quite some time. I’m thinking we are going to move more towards niche social media tools, just as you say, such as a musician’s network (soundcloud? I don’t use it often), writers network (dare I say, like medium.com?) and so on and so forth.
You haven’t mentioned reddit, which I’d argue is one of the last bastions of a huge social sharing conglomerate without obtrusive advertisements, although there are paid users who shill content. And there are advertisements — but not disguised as something else, in my humble opinion.