• A social network for artists - share some stuff publicly, talk privately, have more control over your online self-presentation than you get from Facebook. Arguably demand here is why a terrible site like Ello got traction.
• A social network where you share stuff with friends. No, not Facebook, I don't want to share with everyone I've ever met. I'm talking about sharing with a smaller set of people in a more private space. What if Path was a good idea but was five years too early?
• Local social networks. Facebook groups are doing a non-terrible job here sometimes, but Facebook might be the wrong baseline - makes more sense for this to be public by default. Maybe a Twitterish local social net?

The counterargument being "how many social networks do you want to be a part of?" but seems to me that any time people find a congenial space (cool people, or people with useful information to share) they're eager to jump in. "People" here probably skews toward more active/savvy types, and I don't know how large that segment is, but they're probably the users most likely to make you money if you care about that kind of thing.