Steemit duh.... Lol, slightly kidding, I do however see this absolutely destroying Reddit, it's a mixture of Facebook and Reddit but it's more of an international community style of social media site.

So I don't think facebook's going anywhere, it'll take a hit but there's still a lot of local news for my city, province, and country that I get exclusively or at least before I watch mainstream through Facebook. There's also a few pages on Facebook I find extremely valuable so until I can get that information here, Facebook for me will still be in existence, less prominent that's for sure, but still there.

Reddit however blah I don't think so, just my thoughts.
As for medium, I don't know enough about it to comment so that one I'll stay silent.