The only time I think people should drop out of college is if they're building a company that's becoming crazy successful and making a boatload of money to the point that running that business makes more sense then college.
When it comes to startups there are no guarantees. Risk is high almost always, with some exceptions.
The exception is taking on high risk with your startup.
I can imagine a pretty low risk scenario where you finish school, and start working in a relevant field programming. As you're making money and learning valuable skills you can spend your free time building your business. Unlimited runway and learning from your day job would increase your probability of being successful massively.
The billionaires and facebooks of the world are bad data points to model your future success by.
So at least for him, he was probably pretty sure of his success (a cushy acquisition as the very least).
http://www.thecrimson.com/article/2005/11/1/zuckerberg-to-le...
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.