The Facebook SDK for PHP enables developers to access the Facebook Graph API and use the Facebook Login from within PHP applications. Examples are provided that show how to use the SDK to retrieve a user's profile, post a link to a user's feed, and upload a photo to a user's profile.
Seevl is a new kind of music discovery engine. It allows you to find how artists are similar to each other and which artists are most similar to each other, among other things. It also allows you to comment on the notes about artists within the database. This is all well and good, but better yet, it has the Seevl API, allowing developers to integrate Seevi into their own applications.
Discussion of Google's recent acquisition of Jambool has largely centered around its virtual currency platform, Social Gold. The power behind that platform comes in the form of the Social Gold API. Again, Google has bought a company whose API is central to its success.