Have you ever noticed someone come up in your Facebook “Suggested Friends” feed that made you stop and think, how does Facebook know I’m connected to that person? If so, many people in the online dating scene can feel your pain; according to some users from apps and sites like Tinder and OKCupid, their matches have begun appearing on the site as “people they may know.”

How is this happening, exactly? Facebook denies using data from third-party apps, and both apps also denied providing the information but some critics remain skeptical.

“It is likely that these social network companies are buying data from one another, which means that Facebook can acquire some information on user activity from other platforms,” said Emilio Ferrara, a data science and machine learning professor at Indiana University. “If that’s the case, it would be very easy to cross match… It could also be a coincidence. But I don’t believe very much in coincidences.”

Some other possible explanations: these suggested friends are being pulled in from contact information on your phone, or maybe even from your search history. After all, who doesn’t look up someone they met on the Internet? But no matter how it’s happening, the fact remains that you may one day run into an unexpected face from your romantic past on your very own Facebook page.