Twitter turns 7

Twitter celebrates its seventh birthday Thursday. More than 400 million users are registered on the site, and half of them are active each month. As far as whose tweets are most popular, users with the most followers include:
1. @JustinBieber: 36,011,871
2. @LadyGaga: 35,064,313
3. @KatyPerry: 33,617,790
4. @Rihanna: 28,806,073
5. @BarackObama: 28,377,249
6. @TaylorSwift: 24,961,927
7. @BritneySpears: 24,717,490

Most Re-tweeted tweet: @BarackObama Four more years. pic.twitter.com/bAJE6Vom 811,265 Retweets