I would be a lot more comfortable writing a Facebook app if I knew that I won't be dependent upon their continued good graces. With the platform being open source developers no longer have to host with Facebook. With an open source platform developers can go with Facebook solely to have access to Facebook data rather than to Facebook code and servers.
Another diversionary non-announcement; I don't think Facebook could have prevented anyone from cloning their APIs, and being able to plug Facebook apps into a different social network won't help much since you lose the network effect.
The next step is to free the data.