Your father was right: You will be judged by the company you keep. So when your college pals emerge from their parents' basements to litter your Facebook wall with sexist jokes or embarrassing tales of drunken misadventure, people may think you're like them.
And with good reason: Association does appear to be a reliable indicator of guilt. A 2005 study sponsored by the US Air Force analyzed a Department of Defense database containing simulated profiles of thousands of people, including terrorists who weren't identified as such. Special software sniffed out links and interactions among the profiles. Lo and behold, these associations unmasked the malicious actors.
Such guilt-by-association connections were less apparent pre-Web 2.0. "Previously, you would separate different facets of your life and the people associated with them by time and space," says Judith Donath, director of the Sociable Media Group at the MIT Media Lab. 
"But sites like Facebook collapse groups of people you know from different circumstances." In other words, online social networks merge your barely overlapping social Venn diagram into concentric circles—with you in the bull's-eye.
The only way out is to police your wall, even if that's awkward. Don't be shy about deleting untoward graffiti, eliminating your name from tagged photos, or even asking friends to remove incriminating pics that weren't meant for public consumption. "You might damage a friendship," Donath says, "but that's one of the costs of the collapse of social circles." Then again, you could migrate to MySpace. Nobody pays attention to anything written there.
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