Interesting, though perhaps not fully accurate. At this link you can enter a username that you employ on the web. The engine there then searches 70 websites (Blogger, Digg, eBay, Flickr, MySpace, Tumblr, Twitter, etc etc) to see at which sites that username has been employed - either by you or by someone else.
Several of the sites that gave positive "hits" for one of my usernames turned out to be negative for that name when I went there directly. I don't know if this was a random glitch or a systematic fault.