If we step back a bit, we find that Atom and RSS feeds are already a lingua franca between sites. Virtually every service provides such feeds, giving a way for other tools and services to read someone's outgoing posts. But the fun of social networks is in real-time interaction -- if your updates don't get sent to your friends right away, it's just not the same.
The real-time solution is offered by PubSubHubbub (PuSH). In the PuSH system, a site can subscribe to updates for a feed from a hub server associated with that feed. Whenever a new post is created, the publishing site pings the hub, and the hub sends out just the new posts to all the subscribers.
OStatus is built on that simple base: each participating site produces Atom feeds of updates and uses PuSH subscriptions to send relevant updates to other sites.
The real beauty of it is that at this point we already have something useful, without anything site specific. In fact you can subscribe to someone's public Wordpress feed as an OStatus remote user, and they haven't had to do anything special at all!