You need people to develop and maintain this code, and it touches many aspects of architecture. Keeping it contained to a small team and avoiding any leaks doesn't seem trivial to me. What if some dev not in the know notices the issue and submits a fix or a bug report, what happens then? You introduce a pretty big management liability in your management if you need to protect yourself against your own employees. And what if the employee(s) you chose to throw under the bus in case the trick gets noticed decides to rat you out anyway, for fame, money or to avoid repercussions? How do you even find engineers willing to do something so unethical without raising suspicions in the others?

And again, all that for what? Keep profiles on the small minority of users who bother to scrub their profiles, even though these same users are probably not very good targets for ads in the first place? I don't think it would be very rational for Facebook to try something like that unless they're trying to be evil for the sake of being evil.