Granted, these warnings were usually aimed at those posting photos of naked teenage hijinks, not of chubby, fully-clothed writers. But it seems that even the naked teens of the early 2000s may not find their lives ruined someday by the rediscovery of a long-forgotten online photo.

Take the case of Margaret R. [not her real name], 30, currently the chief financial officer of a boutique consulting firm. When she turned 18, Margaret R. became a nude model for a website that tried to blur the edges between porn and subculture (vaguely like Suicide Girls, but without the drama). One of the site's most popular models, she did numerous photo shoots and videos, and her images regularly migrated to porn aggregator sites all over the Internet.

By the time she quit the business at 21, an image search on her modeling name returned page after page of these photos.

As the quintessential naked teenage girl on the Internet, Margaret R.'s life should have been ruined forever. But earlier this month, when we searched the Internet for her old pictures, Margaret R. and I discovered that they show up infrequently, if at all. Even better, it's almost always the tamest images that have survived, the ones any model might have in a portfolio. She told me that this confirms what she's known for the past 10 years: Her brief career in internet porn has never had any effect on her life since. "Once I quit and stopped being involved there hasn't been anything. I'm actually a little surprised about that, I sort of expected it to follow me more."

Why are the "experts" wrong about our Internet past sticking around forever? Is it that something has changed in the way we access information through search engines in the past 10 years? Or are these two examples—my unfortunate pictures and Ms. R.'s nude pictures—just weird anomalies?

According to Jon Kleinberg, professor of computer science at Cornell University, there's no real contradiction. "The question of why something stays in a search engine's results, and what causes it to show up in response to searches, is a composite of so many things that it's hard to point to any one overriding factor, but a useful working heuristic is that someone on the Internet has to exert at least a minimum of effort in order to keep it around—it has to remain on a machine somewhere that's getting indexed by a search engine, with some sort of way to get to it."

Further, according to Kleinberg, this minimum of effort is pretty minimal. Websites like YouTube and Flickr actively encourage the indexing of their content by search engines, so it's easy to find photos on them that go back many years. But have you searched for an image from your Friendster days? Kleinberg explains, "Our methods for searching pictures are more primitive than our methods for searching text. And pictures are huge—they take up a lot of bytes on a hard drive—so if the site goes away, the consolidation that keeps a lot of things current can also cause a lot of things to vanish all at once, when the effort is no longer being applied."