I think that besides value for advertisers, Facebook should be measured in social value for time spent on site. While the site is useful in connecting people, the same people are spending way too much time on trivial and useless activities.
So "social sites" also have a huge social un-value. Similar to TV.
When people search on Google (or similar) site, they are generally trying to solve some problem, and find information. They are spending time, and getting some value back. The faster they find what they are looking for, the more value for everybody, including Google.
When watching dancing cats on Facebook, they are just spending time. Fill good temporarily, like when drinking "sugary drinks". Then, they get addicted, and get overweight.
Facebook has collected some great engineers, but they should be engaged in solving socially useful challenges. Then, the value of stock may come back up. And that may take some time...