While we are talking about federated social networks, I thought this was a relevant upstart to highlight. Prefpass makes it possible for individuals to authenticate anonymously. The site is yet to launch. And all I know is what I've read at the founder's blog. Here's an excerpt:

Microchunking identity means reducing it to its smallest usable parts. For example, there are some cool solutions that focus on proving that you own a blog URL - that's a microchunk. With PrefPass, we're focusing on letting you tell a site that you're the same person as last time, and that you're associated with some anonymous metadata representing your interests or preferences. That's it. By keeping it simple, we hope to be able to solve some real problems for both sites and users, while making a big change in how much users can control their own data.

Others working on the federation of authentication and identy is SXIP. From what I've read, they seem to be the most advanced commercially. However, I presume these companies will find reasons to work together.