At least a dozen companies offer the service, including startups you've never heard of, such as Adroll (which is on track to do 0 million in annual revenue, according to the company), Criteo and Fetchback, as well as large companies such as Google.
Facebook and Twitter have recently started selling retargeted ads, so that ads inside their social networks can target you based on your activity across the Web.
The question is, how did anyone know you liked that shirt, and are you OK with them knowing that?
Here's how retargeting works: Each time you visit a site, it drops something called a "cookie" on to your Web browser.
That cookie, which can stay for up to a month, is anonymous.
The site doesn't know who you are, or anything about you, but it knows you looked at that shirt.
"Facebook has so much data on everyone already," says Jeremy Leon, a social media strategist at New York-based marketing agency Laundry Service.
"The fact that they now know what you're doing all across the Internet is kind of scary." The good news is you can control how much or little you are targeted (and retargeted) by advertisers.
The practice has become increasingly widespread because it works.Click the question mark, and the ad explains the practice and offers the option to turn retargeting off.Only 3 percent of those who click on Adroll's explainer message decide to opt out, which tells the company that most people don't mind retargeting, says Lauren Vaccarello, the company's vice president of marketing.See also: How to shop safely online Some of the Web's biggest companies, such as Amazon and Zappos, allow users to choose the kinds of recommendations they get in their profile settings.For complete browsing privacy, set your Web browser not to accept cookies (generally located in the settings menu).