Facebook is a free service to use — technically. The price we all pay is in the private information the social media giant acquires from us. That’s why so many eyebrows were raised earlier this year when Facebook announced it was trying to figure out how to interface with your brain.

Many reporters expressed concern right away at the idea of the site being able to read our minds. After all, what would stop it from giving our most private information — our thoughts — to advertisers? In fact, a reporter for The Intercept, Sam Biddle, asked this very question. The reporter pressed Facebook spokesperson Ha Thai for a commitment that Facebook would not use brain activity for advertising purposes, and Thai made no such promise.

This is obviously concerning, and experts agree now is the time to be asking these questions of the site before the technology advances further.

“I would favor assurances that they need to be transparent about what they’re actually recording and how it might be used for advertising,” Dr. Eran Klein, an neurology professor at Oregon Health and Sciences University, told The Intercept. “If it’s subconscious, you don’t have conscious control over what information companies get about you… so you could be targeted for ads for things you don’t even realize that you like.”

It sounds like science fiction, but this kind of technology is coming sooner than you think — and Facebook will likely be the ones at the helm.