Until recently, only a handful of vendors have had access to the Facebook Ads API—a few major tool developers and some folks willing to make a $5M deposit with Facebook. We've been running Facebook ads and building apps for the last 4 years. The breakdown on the Facebook Ads API: It’s not exactly as robust as the Google AdWords API, but that creates an opportunity to build what Facebook has missed, or game the system, depending on whether you’re white or black hat.
Stack Overflow, the very successful question and answer site for programmers, have now released their own API into public beta. The API actually works with all of the Stack Exchange sites, including serverfault.com and superuser.com, with more sites possibly launching in the future. The API is supported by the site http://stackapps.com.