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It's not directly selling the data, but when you target an ad, and a user takes an action on the ad (sometimes that's a click, sometimes it's just loading a beacon in the ad, sometimes the user may need to transact after a click), the advertise can correlate the targetting with the user.

That's not exactly selling the data, but it's close.

Disclosure: former Facebook employee, not in ads except for an ill-fated project that afaik hasn't launched and all my ideas were considered and rejected.