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FORTUNE — Let’s say you’re one of the millions of Facebook users who logs into the site for a minute to reconnect with friends. Maybe you’re taking a break from a big project at work, or settling in for an evening of couch surfing. Maybe, sadly, you have cancer, diabetes, or some other ailment. That’s when you first notice it: A Facebook ad looking for people to take part in a clinical trial — in your age range, with your illness, living in your town.
So how did Facebook know your medical secrets? They didn’t — until you clicked on the ad and told them.
In the last two years, companies like Acurian, a patient recruitment firm that supplies clinical trials with participants, have discovered an inexpensive way to find new patients with social network ads. Their methods are not at all nefarious — they’re not scanning users’ status updates or monitoring the links they post.
Instead, the Horshan, Penn.-based company buys demographic placements that match the requirements of the medical studies it is looking to fill. From there, coincidence, curiosity, and a hunger for cures has compelled millions of Facebook users to click-through the ad and fill out a qualifying questionnaire.