This is the first meaningful Google arrow across the Facebook bow.
Although it will be a good long while before these other sites match the seamless integration of the Facebook open platform execution, this move is really interesting. It will be hard to develop apps that work across such a wide variety of platforms (Plaxo vs. Friendster, for instance), but the beauty of this alliance is the efficiencies created. So what if an app never takes off on, say, Ning or Hi5? This alliance gives access to hundreds of millions users, many times the size of the Facebook universe. The networks that will win will be the ones that figure out how to embrace these apps and make them as elegantly integrated as Facebook has managed to do. Easier said than done, but well worth it.