How did Mark Zuckerberg retain so much equity in Facebook?

From Lee Hower of NextView Ventures...

How did Mark Zuckerberg retain so much equity in Facebook????

The valuation jumps from round to round were orders of magnitude until Facebook hit a $15 billion valuation, resulting in tiny dilutions accompanying each financing.

Note also that with every financing, the Silicon Valley echo chamber thought the investors were nuts to offer such rich valuations. It's now clear the echo chamber was wrong every time; the investors were quite savvy, as a matter of fact.

I don't know exact numbers, but I believe Facebook's equity sales to investors were roughly:

Sep 04 -- 10% sold for $500k. ($5mm valuation -- Peter Thiel, Angels)

May 05 -- 12.7% sold for $12.7mm. ($100mm valuation -- Accel)

Apr 06 -- 5% sold for $27.5mm. ($550mm valuation -- Greylock, Meritech, Founders Fund)

Oct 07 -- 1.6% sold for $240mm. ($15b valuation -- Microsoft)

Nov 07 thru Apr 08 -- 0.9% sold for $135mm ($15b valuation -- Li Ka-shing and European Founders Fund)

May 08 -- no dilution; took $100mm in venture debt from Triple Point Capital

May 09 -- 1.3% sold for $200mm. ($15b valuation -- Digital Sky Technologies)

Jun 10 -- 0.8% sold for $120mm (a blended $14b valuation -- Elevation)

Jan 11 -- 3% sold for $1.5b ($50b valuation -- thank you Goldman Sachs!)

Feb 11 -- 0.1% sold for $38mm ($52b valuation -- really, KPCB?!)