This is great news for developers on one side but bad news on the other: We still have to follow two standards if we want to build our applications for specific social Networks.
Bebo just "licensed" Facebook for its application API and on the other hand we have the powerful OpenSocial community.
We'll see who's going to win the battle - either Microsoft backed Facebook or the Google-Yahoo-AOL powered OpenSocial community. By the way I never understood how Microsoft thought that Yahoo would accept any offer as they are to much good friends with Google.
Facebook itself comments on the Opensourcing of big parts of its platform:
The goal of this release is to help you as developers better understand Facebook Platform as a whole and more easily build applications, whether it’s by running your own test servers, building tools, or optimizing your applications on this technology. We’ve built in extensibility points, so you can add functionality to Facebook Open Platform like your own tags and API methods. We’re also hoping you use Facebook Open Platform in ways we’ve never thought of – just as you showed off your creativity with Facebook Platform, we hope this lets you be creative with the foundation of the platform itself.
Full OpenSocial 0.8 specs
Download Facebook Open Platform