The people in those companies that Facebook bought are not just engineers, they are also people who can see a gap in a market and create a good product to meet the need.

They are not typically the people that apply to for jobs, especially at facebook or respond to head hunter requests.

I can see why facebook needs to buy companies to get those people.

I can also see why they often leave again, since they can't get things done that they want to, in big organisations.