For the developer seeking to experiment efficiently with social APIs, O'Reilly's 2nd Edition of "Mining the Social Web" is a truly outstanding resource.
Author Matthew A. Russell drops the developer right into the sandbox of each social network (Twitter, Facebook, LinkedIn and Google+ are particularly emphasized, as you would expect) with just the right amount of explanation about what's accessible via each dataset, and then clears out all the obstacles so they can start data mining against very clear examples.
Jigsaw, has announced additions to their RESTful JSON and XML API as well as a developer program designed to enable developers to query, share, and resell Jigsaw’s community-contributed database of company data and contact information. They came onto the API scene in June 2008 with their declaration of "Data Independence Day", which focused on providing free access to their database of company information (more details at our Jigsaw API Profile).
You may soon be able to "Digg This" from within your applications. The social news site's API (our Digg API profile) has long been able to list stories and other data. Now it's looking to let developers add content programmatically.