Twitter currently has around 320 million monthly active users, which is a hell of a lot, but that’s only a fraction of its true user base. The problem with measuring Twitter’s user base is that the service’s content is easily accessible to people who aren’t registered with it, and if you were to include the people who don’t have a Twitter account but still use the service on a regular basis, Twitter would have close to one billion monthly active users. That’s not necessarily a bad thing, however, as Twitter is working on a way to make money off of the people that don’t actually have Twitter accounts.