Sometimes, it hits you after you've openly posted comments about a product or a politician. Other times, it strikes you after an online transaction, such as buying an airline or concert ticket, or a Web search about a disease or a dance step. Twerking fans, you know who you are.
Every day, those of us who live increasingly online are adding to a digital footprint others can access. And a study due out Thursday from the Pew Research Center suggests that many of us are increasingly wary of that prospect and are trying to regain a measure of control.
The July survey, based on a sample of more than 1,000 American adults, found that more than eight in 10 said they had taken steps to remove or disguise at least some details of their digital activities.
For instance, nearly two-thirds said they had cleared their Web browsers' history or cookies, the digital files companies use to track your browsing - the source of those ads that trail you from site to site and know about your recent searches.
More than a third had avoided a particular website because it required use of real names. One in four had created a temporary user name or e-mail to cover tracks. And between 10 percent and 20 percent had used fake names, encryptions, or services that enabled them to browse the Internet anonymously.
Are such efforts effective? As recent disclosures about the National Security Agency suggest, there may be no sure way to foil a determined spy agency. But the Pew study shows that most Americans have more mundane concerns about privacy, yet still aren't sure they can protect themselves online.
For example, 42 percent said it would be "very easy" for a company to identify them if they posted anonymous comments about its product, and 37 percent said it would be "somewhat easy."
The truth? That isn't crystal-clear either, concedes study coauthor Lee Rainie, director of Pew's Internet and American Life Project.
"Some companies say that if they're served with a subpoena, they turn over the records. Other companies are more resistant," he said. "If it comes down to a legal showdown or it comes down to a determined opponent, it's probably not possible."
Joseph Turow of the University of Pennsylvania's Annenberg School for Communication said a 1990s law and the First Amendment protect third-party sites from legal challenges over users' comments.
"If you just say a product is bad, it's an opinion," he said. "Yelp has comments like that all the time."
Pew's study found that 28 percent of respondents had tried to hide their activities from advertisers - more than five times as many as those who said they had tried to shield information from the government or law enforcement.
But the study also shows that Internet users, especially younger ones, are focused on controlling access to people within and outside their social circles.
"The research so far has emphasized the ways that people ignore privacy," said study coauthor Sara Kiesler, professor of computer science and human-computer interaction at Carnegie Mellon University. "This research suggests that people have realized there are things that can hurt them - socially, or in getting a job - just by having it associated with their real identities."
Pew found, for instance, that 10 percent to 20 percent of Internet users had tried to hide information from people in their past, from those who might criticize or harass them, from employers or work supervisors or colleagues, or even from family members or romantic partners.
Those findings were no surprise to Asta Zelenkauskaite, a Drexel University media researcher. "People want to decide who knows what about them, similar to what we have in face-to-face situations," she said.
Zelenkauskaite didn't dispute Pew's finding that younger Internet users are more likely to consider controlling their online information a key concern. Quoting University of Amsterdam scholar Mark Deuze, she said, "We are no longer living with media but in media."