The story I've heard to explain this is that it isn't the growth rate so much as the turnover. I have friends who joined Zynga straight out of college (Berkeley) and are looking around for something new a year later, now that they have some vesting in their options. It really doesn't sound like a pleasant place to work -- I was told eighty-hour weeks are the norm.

That said, Groupon has something like 8,000 employees, most of whom were hired in the last two years.