Thinking about platforms and critical mass...
It feels like all major platforms gained traction because they offered some new and desirable feature. Tumblr/Twitter had takes on microblogging, Facebook: social graph, Vine: micro-videos, for example.
So when a platform alternative pops up that does basically all the same things that the original did but it's open-source/decentralized/whatever, it's no wonder it doesn't catch on. Open-source isn't a feature to most.
It is for some (like a lot of us here, me included) but it makes me think that to gain critical mass on a platform, it has to have something *new* that people recognize as a feature. Something that will improve their lives there and then. Once it has users it could potentially expand out and supplant the previous dominant platform.
Or not, what do I know. I've never built a platform. I just grumble at NPM all day.