In case you're not actually trolling, here is why.

Because a change in market price reflects a change in the midpoint of opinion of people on the future value of Facebook, who are willing to put their money where their mouth is with regards to their opinion -- these are not the opinions of idle chatterboxes.

As such, stock price movement in response to news is a very valuable critique of a company's actions. A change in stock price is arguably more objective and valuable information than any random blogger or columnist whose opinion might be published here on HN.

Does that make it clear?