From “De-anonymizing Social Networks” abstract: “…We present a framework for analyzing privacy and anonymity in social networks and develop a new re-identification algorithm targeting anonymized social-network graphs. To demonstrate its effectiveness on real-world networks, we show that a third of the users who can be verified to have accounts on both Twitter, a popular microblogging service, and Flickr, an online photo-sharing site, can be re-identified in the anonymous Twitter graph with only a 12% error rate….” http://33bits.org/2009/03/19/de-anonymizing-social-networks/

…Not that I’ve tried it, but there’s a best-of-omegle chats page that someone put up: http://www.omeglechats.com/