On Twitter: Britney Spears

When considering the most popular Twitter users, Britney Spears is not likely to be the first name that pops into your head. But, believe it or not, the on-again-off-again pop star is Ashton Kutcher's biggest threat when it comes to gathering the biggest crowd of followers. And while Kutcher was the first person to get 1 million followers, Spears has one-upped him in becoming the first twitterer with 5 million followers. The most ironic part of Ms. Spears' massive online popularity, though, is that it's mostly for show; unlike some celebrities' Twitter accounts, which are operated independently by the celebrities themselves, Spears' account is managed by her management. This means that the less-than-regular PR tweets are augmented by even-less-than-regular tweets from Britney herself.