By offering money to leave, we're not actually negotiating what the FaceBook usage value is to the user, but what it is worth to the payee to get that user to quit. (If you're willing to offer me $1M to stop using facebook, that's what I can expect to be paid, even if I would reasonably quit for far less.)
It’s well known that people demand disproportionately larger compensation for giving something up than they would pay to gain said thing.
[0]: https://en.wikipedia.org/wiki/Loss_aversion