at $50B, 0.1% of the company == $50M and 0.01% = $5M not a bad payout vesting over four years, when the stock is probably going to go up... no wonder Google is having trouble keeping talent!
EDIT: This might shed more light: http://www.quora.com/Is-this-a-good-offer-for-working-at-Fac... (that says 125k options)
Also, stock options need to have a hardly-discounted exercise price attached. That is, an employee that gets a stock option package reflecting shares worth $500K will have to shell out (at least) $450K to exercise them when the time comes. So based on this valuation, if facebook is worth "only" $70B in 4 years, the profit is going to be $250K or ~$60K/year. Nothing to sneeze at, and definitely a nice bonus -- but not more than that. And if facebook is worth $30B at the end of 4 years, today's stock option grant is worth virtually nothing.
Pay attention to tax laws. If you are well off, that is your single largest expense.
edit: speaking, of course, of a newbie who was hired very recently with the 10 digit valuation in mind