I'm sure any pre-IPO stock issued to their employees, while not being a really rewarding thing right now, is going to pay off big in a relatively short timeframe... see: http://techcrunch.com/2010/09/01/google-making-extraordinary...

In not any particular order,

1. Good quality true "Web scale" work.

2. Top benefits (and I suppose a decent paycheck too).

3. Smart peers.

4. Realistic chances of hitting the IPO/Acquisition jackpot through equity.

Whats not so great at working there?

It's rewarding when your work is used by 500 million people.

I like their philosophy that you can make changes and push them often so that you do see immediate gratification that your work means something. A typical non-startup developer's work could take years before it ever hit a desktop.

While the stock options that others mention is a motivator, typically companies use options to buy down your salary. You would make $120k, but, we're going to give you $90k + $30k in options that could be worth more than $30k in a few years. Pre dot-bomb, extremely greedy employees would work for a .com, trade huge amounts of salary for options and live frugally until the IPO. I can think of one guy that worked for Real Networks for a few years that took 66% of his pay in options which were underwater when his options matured.

Options are a payment type and are a gamble on the employee's part. The employer believes that shared ownership will also motivate employees, which makes those options worth more.

They're dealing with very interesting scalability/complexity problems, and they're currently pre-IPO.

It's not for the money - any of the talented folks could go work at a hedge fund and make 10x. The secret sauce is that it's a fun culture, great product, and the company has huge potential to innovate into new areas. And you get fed.

There's a potential with Facebook that isn't there with many other companies. It has an enormous social graph but hasn't found a way to fully unlock its value. The challenge is to help find Facebook's business-model and technical equivalent of Adwords, if such a thing exists.

Thrill of a start-up but with the "security" of some serious investment and traction. It's the wild-west and they encourage experimenting by hiring the smartest people they can find. 15-20 years ago you might have asked the same thing about Microsoft.

Run a few basic, even worst case, valuation scenarios and I think you'll answer that question for yourself.

You get to tell your friends you work at Facebook and they get to be like oh sweet and they say what are you working on and you get to say oh the billing platform for virtual goods and they get to say like whoa that is so exciting!!