There's ones in the works. Diaspora uses an amalgamation of protocols (webfinger, ostatus, a few others), Appleseed uses a homegrown protocol called QuickSocia

There's ones in the works. Diaspora uses an amalgamation of protocols (webfinger, ostatus, a few others), Appleseed uses a homegrown protocol called QuickSocial, but is designed to be protocol agnostic, and there are a number of participating projects (spearheaded by StatusNet) working on a federated social protocol. Who will win out is anybody's guess, but there's solid work progressing in that arena.