"The challenge is to recommend missing links in a social network. Participants will be presented with an anonymized, directed social graph from which some edges have been deleted, and asked to make ranked predictions for each user in the test set of which other users they would want to follow."

When has this kind of data ever been successfully anonymized?

From Netflix's attempt: http://www.securityfocus.com/news/11497 (First link I googled, but has the gist)