Who's to say the next horseman won't resist the advances from the big Four? Zuckerberg retained control for a long time, and it's not inconceivable that the Next Big Thing won't get acquihired.
I think Snapchat is a good example. The founder(s) still have a controlling interest in the company (or they did up until the IPO, at least). They want to build a company rather than sell it off. That's why they IPO'd, for instance.