Facebook's stock has dipped below $20, an all-time low and about half its price when the company first went public. The stock price has been trending downwards since launch, when glitches made trading difficult or even impossible at times. Now, Facebook seems unlikely to live up to its massive valuation. A recent Facebook earnings call revealed revenue that barely met expectations, and growth was slower than in previous years. The company also offered no guidance for future revenue.

Facebook still has an impressive active user base, dominating the social network market. But mobile use is growing rapidly, forcing the company to adapt its advertising strategies to an area it's less familiar with. With 900 million users, Facebook has already saturated the market in many parts of the globe, and it earns less revenue from its fastest growing audiences in Asia. Despite Facebook's size, its $104 billion valuation was quite a premium given its revenue and trajectory, and some analysts have said a fair value for the stock would be between $16 and $20. That means the price could still have further to fall.