"The news that Twitter has taken the first steps towards a stock market flotation has triggered a predictable storm of speculation about the valuation of the company. How much is a corporation with 200 million monthly users actually worth? How does it compare with Facebook, with its billion users? The answer is: nobody knows. But that doesn't matter because it's not the important question. Although Twitter and Facebook are categorised as social networking services, in fact they are as different as chalk and cheese. And, of the two, Twitter is more important in one respect: its impact on the arena in which societies discuss their political issues."

http://www.theguardian.com/commentisfree/2013/sep/14/twitter-flotation-facebook-politics-social-network?CMP=twt_gu