Mark Zuckerberg, Facebook's 25-year-old founder and CEO, had a pretty good year in 2009. His company grew beyond expectations, and he made a lot of money. Still, there was one thing he couldn't get: Twitter. In late 2008, Facebook approached the trendy microblogging site about a potential merger. Zuckerberg offered Twitter company stock — about $500 million worth — but in March 2009, Facebook confirmed the rumors that Twitter had turned down the offer. What was the problem? No one was sure how much Facebook stock is actually worth. Although Facebook's net worth has been estimated by analysts at $15 billion, it's a private company with no easily trackable stock price. No one knows the true value of all those millions of friend requests, and Twitter wasn't about to volunteer to find out.