From a blog posting at status.net on distributed networks of the "social web" - very interesting as it looks like someone has taken a pragmatic view and actually reused some of the existing simple protocols :

Luckily for us, there have been a number of advances in the distributed social Web in the last two years. In Fall of 2009, we started re-designing OMB. We abandoned any attempt at backwards compatibility, so we were really able to come up with a system built of shiny new components that make more sense.
These components are:

Atom or RSS for feeds. These standard feed formats make it easy to include rich information like location, thread context, and HTML formatting in status updates.
PubSubHubbub for real-time. PubSubHubbub (or PuSH) is an HTTP-based protocol for pushing feed entries from publishers to subscribers. It lets people on different servers receive status updates in real-time (or "real-enough time").
Salmon for replies. Salmon is an excellent, distributed, secure protocol for sending messages "upstream" -- from subscribers to publishers. And since it uses Atom or RSS elements to structure those messages, it's easy to include lots of rich data in these messages.
ActivityStreams for social events. There is a lot of information in a social network that's not explicitly a status update: follows, faves, repeats ("retweets"). We use ActivityStreams to encode these events, which then either flow through PuSH out to a network of subscribers, or get posted through salmon to the recipients. This lets sites on both sides keep track of followers and friends with a minimum of data transfer.
Webfinger for discovery. We needed an easy way for people to say, "Follow me!" Webfinger gives people identities available across the Internet that look like email addresses. We use this for discovering people on the network; you can follow me at evan@status.net.

Putting these pieces together, we were able to make the new, distributed social networking system that's part of StatusNet 0.9.0. It's great; flexible and simple and high-performance and fun and easy to use.
We wanted more sites to implement these protocols so the network becomes even more valuable. Some parts have been implemented already by sites like Google Buzz, LiveJournal, WordPress.com, and Tumblr. We want to make sure that sites that have taken those first few steps make the next ones to build a truly distributed social Web.
So we've put a name on this suite of protocols: OStatus. We want people who are looking for a way to distribute status updates to know that there's an easy, standard way to go. We think that the more OStatus participants, the better the network becomes.
We've published the OStatus suite's definition as OStatus 1.0 Draft 1. It's not a long spec; it depends on the great work that's been done on the component protocols, with a teensy bit of glue to make everything even out right. We think it's a natural and obvious way to use these protocols together, and we hope to see more sites on the OStatus network moving forward.

[From Understanding OStatus | StatusNet]