It would be more accurate to say that Facebook decided that it would rather go public than stay with <500 owners. It was hardly "forced", since there's nothing forcing you to have more than 500 owners of a company!

If you keep a handful of owners and pay employees with cash-money instead of equity, you can stay private forever. Mars, Inc. was founded in 1911, has revenue ten times as much as Facebook, and has never been forced to go public.