http://www.theatlantic.com/technology/archive/2011/08/why-facebook-and-googles-concept-of-real-names-is-revolutionary/243171/

To quote few lines from the post:
"I can understand why Google and Facebook don't want this to happen. It's bad for their marketing teams. It generates social problems when people don't act responsibly under the cloak of their assumed identity. It messes up the clarity and coherence of their data. And maybe those costs do outweigh the benefits pseudonymity brings to social networks."