The datasets available on the website were mostly collected (scraped) for the purposes of our research. Social networks: online social networks, edges represent interactions between people. Communication networks: email communication networks with edges representing communication. Citation networks: nodes represent papers, edges represent citations. etc.
Twitter and Infochimps have figured out what they want to do, and the Twitter census data is back up. It's no longer a measly 2.7 million users anymore though. The population has grown to 35 million.