[27182818284]

Although true that they couldn't have been sure, they were better off than 99% of dropouts. Zuckerberg was just taking a leave of absence and planned on returning. Before entering Harvard, he had already been noted on Slashdot and in PC Magazine. Gates entered with a nearly perfect SAT course and took the legendary Math 55 course. (http://en.wikipedia.org/wiki/Math_55)

In other words, they were already exceptional before dropping out. That key point gets lost a lot of times. I think the exception is probably Jobs and WoZ.