I hadn't seen this site before but it looks kinda interesting:

I looked at these guys some time after I came up with the idea of socialmaps for m-by-mc. They were one of the reasons I thought it might be foolish to build something of my own and partially why I lacked the enthusiasm to put in the effort ... but after giving it quite a bit of thought what we're trying to do here is sufficiently different from what they are doing that it might be worth it.