As I understand it (which is to say, poorly), one of the more pressing reasons is this: the SEC requires companies with 500 or more shareholders, whether or not they are publicly traded, to publicly disclose financial information - exactly what financial information, I'm not sure, but the types of things that publicly traded companies are required to disclose. So, since they're going to have to deal with the regulatory/disclosure stuff either way, it often follows that they may as well take advantage of some of the benefits you listed (rewarding investors & early employees, add'l cash, etc) by going public.

While googling to double-check myself on this, I discovered that the SEC is considering raising the shareholder limit thanks to Facebook. And of course, I'm sure FB is weighing this stuff very carefully. Personally I'm really skeptical about the choices that might get made at Facebook out of concern for their obligations to shareholders as a public company. I used to say that the IPO would be the beginning of the end for them, but I'm not so sure anymore. Last I remember, though, they were already close to the limit of 500 and may be considering going public next year, so we might find out soon enough.

I'm no expert, but I don't think they really need to. I feel that many companies do it just because it's "what you're supposed to do".

Some credit should definitely be given to Zuck. He's not here for the money, he's here for the thrill. Definitely a little Jobs in him.

I think that's overstating it a bit. I mean, he's already a billionaire, so the thrill is really all he has left to seek.

In high school, he refused to sell his company for $2 million...

Eventually employees will want to cash out. Facebook has been giving out RSUs for several years now and there's no easy way to liquidate RSUs without an IPO.