Dilution doesn't really apply to these employees. They don't care about their amount of ownership in the company. All that matters is how much stock they get and what they expect the stock to be worth at IPO. If they've received 10,000 shares and the IPO is at $100, then they're millionaires by the end of that day (on paper, at least).
...at the end of the employee lockout period, you mean. =)
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.
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.
