As we wrote last week, Facebook is turning parts of its application platform open source, the company announced today. It’s available here for download.

This comes a little more than a year after Facebook Platform first launched to allow third party developers a way to get their applications directly onto Facebook. The company says more than 24,000 applications have now been built on the platform and more than 400,000 developers are building these applications. 140 new applications are added to the directory each day. “Nearly all” Facebook users have added at least one of those applications.

Facebook Open Platform is licensed under the Common Public Attribution License (CPAL), except for the FBML parser, which includes Mozilla source code, which is licensed under the Mozilla Public License (MPL).

Facebook says they’re doing this “to give back to the developer community.” That may be somewhat true, but the key reason for fbOpen is to compete with OpenSocial, the Google/MySpace/Yahoo/AOL led open source competitor to Facebook Platform.

Competing social networks, including the still-larger MySpace, are lining up against Facebook via OpenSocial. This is their way of responding.

It may be too late. Tellingly, Facebook was unable to line up any partners to add to today’s announcement, although some social networks we’ve chatted with say they will almost certainly implement it in the near future.

More details here.