Writing a modern OS is almost impossible from scratch, but there's nothing preventing you from hiring enough people and making Windows or Azure.
But the problem with Facebook and Google is that they started by scraping stuff for free and without limitations, but if you try to do the same thing with them they bite you with Terms of service, paid data graphs and API quotas. How do you build a index of businesses/venues and people? No matter how many people you hire you cannot achieve the same snowball / network effect.
Instead of breaking them up, make them release open data.