Urbit, under all their jargon, is a base for a federated social network. Sort of like Diaspora. (Right now, they say they have chat, so it's sort of like IRC.) They recognize that a big problem with social networks is spam. If you can freely create user IDs, it's hard to stop spammers. But if user IDs cost money, then spam blocking costs spammers money (their purchased online identities lose value) and spamming is no longer cost-effective.
So their solution is to create a market in user ID space. It's not clear if this will work out, but it's something. If you buy an ID, you own it, in the Bitcoin sense. That is, you're not at the mercy of some service provider, and can freely move from one service provider to another without their permission. (I think this is right. Not entirely sure.)
It's not clear if this will work in the social space. They have the underlying machinery, but nobody has built the Facebook or WhatsApp or Dropbox killer on top of it yet, so it's useless to end users at this stage. The concepts are interesting; it's good to see people trying original ideas.
If someone builds an easy to use social application on this and gets some services to host it (like Wordpress hosting), this could get actual users. Right now, it's a developer toolkit.