By creating a distinctive username—and reusing it on multiple websites—you may be giving online marketers and scammers a simple way to track you. Four researchers from the French National Institute of Computer Science (INRIA) studied over 10 million usernames—collected from public Google profiles, eBay accounts, and several other sources. They found that about half of the usernames used on one site could be linked to another online profile, potentially allowing marketers and scammers to build a more complex picture of the users.
Experts say users should avoid websites that openly publish their data. "It's not surprising that people use the same username in different places," says Avi Rubin, a professor of computer science at Johns Hopkins University who is currently on sabbatical as a Fullbright Scholar at Tel Aviv University. "What's important is that people pick different passwords for different Internet sites, and that knowledge of their password for one site does not provide any useful clues toward deducing their passwords on other sites."
Folks use exactly the same techniques to "out" anonymous bloggers/commentators. One of these usernames terminates in a real name or information on where the anony lives or works, and Hey Presto! someone is banging on their door with legal papers. Unusual signature lines are just as bad as usernames, incidentally.
And, in what I am sure is a coincidence...