More than nine million people are now actively following American President Barack Obama on Twitter.

His microblogging account is the fourth most followed in the world (behind Lady Gaga, Justin Beiber and Sina's Yao Chen).

Officials running President Barack Obama's Twitter feed are rousing up support for his 2012 re-election campaign tweeting, "Our challenge for you: Bring 50 new people into this campaign for President Obama's 50th birthday. Are you game?"

Twitter's most followed tweeter Lady Gaga is keeping fans up to date with her touring plans in Sydney, Australia. The songstress let her fans know she is "So excited to perform on Australia's TV Show 'A Current Affair' with Tracy Grimshaw. It's on Channel 9 at 6:30pm tonight so don't miss it! x"

Ellen DeGeneres surpassed the 7 million followers milestone this week, but was overtaken by South Korean actor Jung Kyung Ho, whose follower count rocketed past 7.1 million followers during the same period.

Chinese actress Xie Na is making her way up the ranks of most followed microbloggers too. Her 6.981 million follower count was just enough to put her ahead of Colombian singer Shakira, who had 6.980 million followers to her name.

Interesting fact: Weibo's notable list of English-speaking users now includes British band Radiohead, singer Sean Kingston, Microsoft founder Bill Gates and actor Tom Cruise.

In September 2010, Twitter had 175 million registered users, while Sina reports that Weibo had more than 140 million registered users in May 2011.

Most popular people on Twitter:

  1. Lady Gaga ( 11,543,798 followers)
  2. Justin Bieber ( 10,937,256)
  3. Barack Obama ( 9,078,503)
  4. Britney Spears ( 8,486,587)
  5. Katy Perry ( 8,272,244)
  6. Kim Kardashian ( 8,243,924)
  7. Ashton Kutcher ( 7,144,058)
  8. Ellen DeGeneres ( 7,016,281)
  9. Taylor Swift ( 6,993,972)
  10. Shakira ( 6,980,156)

Most popular people on Sina's microblog (Weibo):

  1. Yao Chen (9,562,908 followers)
  2. Dee Hsu (8,682,462)
  3. Kevin Tsai (7,753,335)
  4. Zhao Wei (7,339,265)
  5. Jung Kyung Ho (7,123,491)
  6. Xie Na (6,981,048)