Zuckerberg was lucky with a big jump on valuations on Series B Peter Thiel didn't invest because of the runaway valuation, later he regretted [1], and being a "Solo-Founder" :) if standard funding history with 2 co-founders Zuckerberg should have IPO with 5%~10% stake.

[1] "Biggest mistake ever was not to do the Series B round at Facebook," http://www.businessinsider.com/peter-thiels-bad-investment-2...

Equity in Startups - Cap tables at IPO

https://news.ycombinator.com/item?id=2134116