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.
When your company is where Snapchat was at the time there's a good chance you've already taken millions off the table as part of a round. Even if not, when your company has achieved that level of growth, you can feel fairly confident that you will walk away fairly wealthy in the end, and it's more interesting to continue running your own company than it is to be rolled into someone else's.
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...