People were also shocked when Zuckerberg turned down $1+ billion offers. Hindsight is 20/20.
If things seem to be going relatively well, and you've already taken $10-20-50m off the table, what's the point in selling? At that point it's only really cash in a bank account that you then need to manage and reinvest. Obviously, if you feel the wheels are going to come flying off, sell.
Same for a college degree. If you're a self motivated person and a learner, you don't need it. But if you need your hand held, you won't achieve much. Degree or no degree. More than anything this has to with the person and his/her work ethic and intellectual curiosity.
That said, a social network centered on photo sharing and a slightly more handy version of email or event sharing is definitely not one of these high impact ventures in my opinion, so yes, I would have sold that long ago and went to work on something more meaningful.