Do we honestly believe there would be any less Facebook users if the clause had actually been included in time?
Not with the prevailing TL;DR attitude. It would indeed not have made a blind bit of difference from the users' perspective. However, from the FB side this is a problem as it had not declared this intention and thus ended up on the wrong side of Data Protection laws.
Not that they care much - I don't think the fine will even register in their accounts.