Facebook is open source! This is an incredibly good news.
The license they chose is interesting.

Facebook Open Platform (except for the FBML parser) is licensed under a Common Public Attribution License (CPAL), which follows the Mozilla Public License (MPL) with two additions:
1. That you include attribution to Facebook on any modifications.
2. That network deployment, or making modifications available over the network, counts as distribution, which makes the license appropriate for Web services.

I would have bet for an Affero GPL instead, what is the difference and why did they choose CPAL instead of AGPL?

About motivation, well, t-shirts ;)

If you’d like to contribute to Facebook Open Platform, please sign and return our Contribution Agreement. We’ll evaluate any submitted patches or features to decide whether they’d be strong inclusions into the overall Facebook Open Platform release. If we incorporate your changes, we’ll send you a t-shirt!