I guess if you're a Lars Rasmussen-class talent you're going to get some special class of stock to entice you to come over, but what about the rest of the new hires?
By most estimates there are 300 million shares of Facebook generated in various capital rounds and whatnot. From my experience, if you're coming into the game this late you'll make some money with an IPO but only the founding members and VC groups will make fuck-you levels of wealth on the offering.
That's why I said it's on paper.
These employees have the opportunity to possibly make 10 to 20 years of their current salary in a day. Most people don't need more of an incentive than that.
If this doesn't sound questionable enough, keep in mind that this is the only retirement plan they offer to employees and that upon leaving the company employees are required to sell back their shares.
Maybe it's totally legit, but it always struck me as very, very questionable.