Facebook Inc. founder and CEO Mark Zuckerberg became a very young billionaire when Facebook went public almost two weeks ago, but now that the company’s stock price is declining, he is no longer one of the 40 richest people in the world.

At the time of Facebook’s initial public offering, Zuckerberg was worth $16.2 billion, but as Facebook stock continues to decline on fears that mobile advertising will not be as profitable for the company as expected, so does Zuckerberg’s wealth. Zuckerberg is now worth about $14.7 billion.