Moreover, I think it's interesting to think if Facebook would be different had it been founded by post-graduation Zuckerberg. Better? Worse? I bet the Facebook experience would be richer had Zuckerberg been more mature when he crafted the initial vision.
Also, how can someone consumed with an idea finish school? It is that singular purpose that drive Zuckerberg to succeed. You can't just put that on hold and finish up a degree.
The uber-point is really, people who are bright enough to found $1B businesses are also bright enough to realize that you don't leave college until you have the idea moving along a good path.
To put it another way, are there any $1B SW companies started by people who either didn't go to college or dropped out of college before they had started creating product for what would be this company? I honestly can't think of any (but that doesn't mean there aren't any -- and it should be easy to enumerate since there's probably only 20 or so billion dollar SW companies).
And I think half credit for Twitter. I don't know much about Twitter, but it appears Dorsey was still in college when they started Twitter.
But in any case two pretty good examples.
It's actually really hard to drop out or fail out of an elite college - I tried a couple times when I was still in undergrad. They always want you to take a leave of absence because then you don't show up in the dropout statistics that drag down their U.S. News rankings. This is why it's not really all that risky to drop out of college; you can always go back and pick up where you left off, but you'll have a much better idea of why you're there.
I was definitely thinking about Evan Williams for Twitter. He dropped out of college because he always knew he wanted to be an entrepreneur, but had no idea what sort of company he would start. I don't really know about Dorsey and Biz Stone's backgrounds.
It is always assumed that success and failure in business corresponds directly to skill. I doubt this assumption and suspect luck has a lot to do with it. Once you get your first success earning you several million, many doors open to you.