WHEN Facebook goes public later this year, Mark Zuckerberg plans to exercise stock options worth $5 billion of the $28 billion that his ownership stake will be worth. The $5 billion he will receive upon exercising those options will be treated as salary, and Mr. Zuckerberg will have a tax bill of more than $2 billion, quite possibly making him the largest taxpayer in history. He is expected to sell enough stock to pay his tax.

But how much income tax will Mr. Zuckerberg pay on the rest of his stock that he won’t immediately sell? He need not pay any. Instead, he can simply use his stock as collateral to borrow against his tremendous wealth and avoid all tax. That’s what Lawrence J. Ellison, the chief executive of Oracle, did. He reportedly borrowed more than a billion dollars against his Oracle shares and bought one of the most expensive yachts in the world.

If Mr. Zuckerberg never sells his shares, he can avoid all income tax and then, on his death, pass on his shares to his heirs. When they sell them, they will be taxed only on any appreciation in value since his death.

Consider the case of Steven P. Jobs. After rejoining Apple in 1997, Mr. Jobs never sold a single Apple share for the rest of his life, and therefore never paid a penny of tax on the over $2 billion of Apple stock he held at his death. Now his widow can sell those shares without paying any income tax on the appreciation before his death. She would have to pay taxes only on the increase in value from the time of his death to the time of the sale.