I don't understand why anybody would ignore a >$100mio offer if he's not already a millionaire. The difference between not being a millionaire and being one is too huge to ignore, even if the company is undervalued by the offer.
Evan Spiegel passed on a 3B offer for Snapchat, and he owns 14% of the company. Today despite the drop, Snapchat is still worth $18B and Evan is worth 4B. And he also gets to stay CEO and control the product, which is a huge benefit of staying independent.
Citation: http://www.businessinsider.com/snap-stock-price-evan-spiegel...