From Techcrunch:

When asked about how an advertiser should view Twitter’s liberal pseudonym policy versus that of Google+ and Facebook, both of which require users to sign up with their real names, Costolo said that the primary difference between how the companies make money is that is that brands like Virgin America pay Twitter when someone, whether it be @bozo123 (his example) or @johnsmith clicks on a link. “We’re not wedded to pseudonyms. Other services are declaring that you have to use your real name, because it will help them monetize. We want to serve our users first.”

Just remember, the advertisers are the users. You are the product.