If one follows the "it's worth whatever people are willing to pay for it" model you'll get burned just about every in the market long-term. It's simply too vague. That is what is involved in bubbles and it never lasts. Fundamental valuation almost always comes back with a pin to pop the bubbles.
You need to follow at least the most basic of fundamentals -- the most common used valuation methodology is P/E ratio. Start with that and dig further using the other popular metrics (growth rate, EPS, PEG, EV, EBITDA, etc)
The reason FB stock tanked is because their revenue growth is decelerating. Then you combine that with the transition into mobile (and lack of proven monetizing methods), Zuckerberg refusing to give guidance and it's a major concern to investors. Stock market is about paying for future growth and it doesn't appear to be there for FB.
