**UPDATED** From the 'read the fine print' files:

If the rumor (and TechCrunch) are to be believed - Facebook is going Open Source. Kinda/sorta.

The root of the leaked news is that the Facebook Platform which allows third-party developers to build applications for the social utility's users will move to an Open Source license. The open sourcing of the Facebook Platform is not (unless I'm missing something) an Open Sourcing of Facebook itself - which though it is built on open source components is not open source itself. That is - don't expect to be able to simply download a tar ball and build your own Facebook next week.

The open sourcing of the Facebook Platform fundamentally means that the developers will be able to freely extend and integrate with Facebook in an unobstructed manner.

IMHO I think all developer tools that extend platforms should be open source - it's good for developers and it's great for the platforms as it removes barriers to entry.

The Platform release won't be Facebook's first official foray in releasing open source code either - last year Facebook open sourced Thrift which Facebook defines as,"a software framework for scalable cross-language services development".

*UPDATE* just got an email from Facebook clarifying their plan:

We're working on an open-source initiative that is meant to help application developers better understand Facebook Platform and more easily build applications, whether it's by running their own test servers, building tools, or optimizing their applications. As Facebook Platform continues to mature, open-sourcing the infrastructure behind it is a natural step so developers can build richer social applications and share what they've learned with the ecosystem. Additional details will be released soon.