At YourTrove, a lot of what we do involves ingesting social content via APIs and then regularizing that data within our system. For a lot of data, this is pretty straightforward. For example, while Facebook and Flickr might return different meta data, or name fields differently, no one disagrees that a photo is a photo. This is true for essentially all uploaded binary user generated content.
ProgrammableWeb has been tracking Twitter mashups since the first one was added in December, 2006. Looking at the number developers have added to the database, one thing is clear: 2009 was a huge year for Twitter and apps built upon it.