The reason why the co- founder of Facebook is so rich, while joining the site is free is because Facebook generates revenue through advertising. The site takes your private information and markets it to companies which manipulate their advertisements to appeal to your interests.
Just because we don't pay for Facebook doesn't mean its free. In fact it's very expensive but is paid for by advertising. It's like when people say something about something the government pays for being free to us. Of course its not really free. Tax payers are paying for it.
One site says Facebooks share of all sales last year was one billion dollars. Total sales via Facebook was 3.7 billion. Their initial public offering or IPO before going public on the stock market was five billion and once it goes public experts believe the company will be worth around 85 to 100 billion.