Facebook did try to implement something like this with their updated Groups functionality, but it still hasn't caught on. There's definitely a wide open gap for

Facebook did try to implement something like this with their updated Groups functionality, but it still hasn't caught on. There's definitely a wide open gap for somebody to make a usable circles-like social network, but it would probably need to use the Facebook social graph as a starting point to gain any traction, and would probably want to aim for an exit strategy that ends with either Google or Facebook buying them out. I tried working on a similar idea a couple years ago, but dropped it because I couldn't find enough of a draw to make even myself switch over from Facebook to my network, let alone other people. I wish the OP best of luck in executing this idea better than anybody else has managed to yet.