When it came to monetizing search, Google was lucky in that someone else already figured out how to do that (GoTo.com). No comparably effective model exists for monetizing high traffic websites (social media or otherwise), despite many attempts over the past two decades. Not even close.

If Zuck took Facebook public 5 years ago, they would have gotten much less money, and they would have been distracted by investors calling for them to focus on building out that business model rather than attracting users - and Zuck may have lost controlling power in the company (less money + less users = less leverage for Zuck).