Assuming the first round of $500K was @ $2.5M valuation, giving those investors 20%. So the founders are left with 80%.
Further assume the 2nd round of $7M @ $20M valuation, giving those investors 35%. So the founders are left with 45%.
Further assume the 3rd round of $40M @ $500M valuation, giving those investors 8%. So the founders are left with 37%.
Also assume that the employee stock options pool is worth 10% of equity. Founders left with 27%.
There are two founders, according to Crunchbase [1], so assuming a 50% split for each founder, each founder has 13.5%.
At $1B, each founder walks away with a cool $135M in cash + Facebook stock (which is likely to appreciate significantly in a few months) - which is likely another reason they chose to go with FB as opposed to Google.
Not bad for 2 years worth of work.