Facebook CEO Mark Zuckerberg had a tough week: His sister went to work for his top competitor, Google, his company's stock price hit a new low and now he has dropped out of the top 10 tech billionaires list.

Zuckerberg's fortune fell by $423 million on Thursday to $10.2 billion. That puts him $400 million behind James Goodnight, the co-founder of SAS Institute, the new number 10 on the $21 billion when the company went public in May. Zuckerberg owns 503.6 million shares of his company including 60 million options with an exercise price of $.06 a share. He also has $150 million in cash and other liquid assets, Bloomberg reports.

Facebook's stock price on Thursday was down 47% from its $38 debut. On Friday morning, however, the stock was up about 2%.

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