The purpose of the fine is to discourage FB from engaging in illegal and unethical behavior. The problem here is that FB sees a $3B fine as the cost of doing business, since they surely made much, much more than $3B engaging in these invasive, user-hostile tactics. If they know that regulators will only come after them for an arbitrarily low percentage of revenue generated from these activities, why would they stop doing them?

If a punishment no longer deters bad behavior, then it's no longer a punishment.