Unlike Facebook or Google+, which have been pushing a real names-only policy, Twitter doesn't give a rip:
In a recent open house at the company, CEO Dick Costolo talked about how the service doesn’t really care what your real name is — all it wants to do is connect you to the information that you care about. And if that information happens to come from a “real” person, then so be it; but if it comes from a pseudonym, then that’s fine too.
On Twitter, no one cares if you're just an avatar. Google and Facebook execs have recently argued that real names are important, to curb bad behavior. But Twitter's architecture curbs bad behavior too: Jerks who post Spam, hate, or flames lose followers, or can be blocked entirely. And since Twitter influence depends on earning followers, users have an incentive to be civil and Tweet interesting things, whether they're using their real name or not:
The reason why services like Klout have been gaining steam is that advertisers and marketers are looking to build a “reputation graph” that they can tie to the interest graph they get from watching behavior on social networks. They need to know not just what is being talked about but who is saying it, and whether they are influential. Does their real name matter? Not really. Did anyone care that Perez Hilton used a fake name as he built a small media empire under the noses of the mainstream media? No. Advertisers certainly didn’t care, because he had influence in the markets that they were interested in.
This points out another shortcoming to a "real name" focus. Supposedly, the real reason Facebook and Google want real names is that they're valuable to marketers. But when you're using your real name, you're actually inclined to share less of what you buy and do with your time. (Thought experiment: Name ten things you own or activities you do that you don't want everyone in your Facebook network knowing about.) In this way, pseudonyms are probably more valuable to marketers, since they encourage users to be more open about expressing in public what they really want.
This isn't to say there's no place for real names online. But we are fast reaching a point where Facebook will enjoy 50% market penetration of all 2 billion Internet users in the world. If past performance is a guide, Facebook growth will finally plateau at that point, just like it does when it reaches 50% of a country's online population. At that juncture, many will look for the best place for maintaining an identity beyond Facebook, and for many reasons both political and personal, they'll likely look first to Twitter, for the best alternative.