According to the Web site, Facebook Open Platform includes "the API infrastructure, the FQL parser, the FBML parser, and FBJS." All components except the Facebook Markup Language parser are licensed under the Common Public Attribution License, and the FBML parser is available under the Mozilla Public License.
If you'll recall, SocialText walked the CPAL through the Open Source Initiative approval process last year after OSI chairman Michael Tiemann vowed to be more protective of the open source label. The license requires that those using code to "include attribution to [the originator] in any modifications." Because it explicitly provides that network deployment of modifications licensed thereunder constitutes distribution, the CPAL also applies to Web services.
Arrington points out that though Facebook's stated reason for the release is "to give back," the more predominant motive isn't all that altruistic -- namely, competing with the Google and Yahoo collaboration called OpenSocial.