Once you are a member, you can keep links to all of your friends that are also members. From those links, then, you can see who the friends of your friends are (possibly increasing the circle of your own friends), join and participate in online communities, and more. Nothing incredibly original, but the implementation of it is pretty compelling. It’s been fun to play with, anyway.
It has a lot of potential; I’ve already joined the orkut Ruby community, and the D&D community, both of which are quite large (almost 200 members at the time of this writing). I’ve even found a few people that I knew, just by searching for them—kind of fun!
It’ll be interesting to see where this whole thing goes in the next few months.