From VentureBeat:

"There’s a growing recognition by Zuckerberg that 'all human
behavior boils down to incentives.' That’s reflected by Zuckerberg’s
efforts to motivate each person according to what gets them excited:
For engineers, that means giving them the best product to build, but
for business executives, it means big financial incentives....At
Facebook, if you excel in meeting your project goals, you can double
your stock option count in 1.5 year 'refreshing' cycles. This individual
multiplier is added on to another bonus that is based on how well the
company does as a whole, in terms of user and revenue growth. But here’s the tough part: Managers have to force-rank their employees. The more Machiavellian managers make a point of telling their employees where they stand. If you’re tenth on of team of 10, you’re hanging on by a thread, and you know it."

http://venturebeat.com/2009/10/02/mark-zuckerberg-the-evolution-of-a-remarkable-ceo/