As Twitter this week hit and exceeded its 200-million-tweets-per-day milestone, the social network revealed the most popular topics to circle the microblogs during the first half of the year.

Every five days, around a billion tweets are catapulted around the interwebs.

The billions of 140-character posts contain "[e]verything about every topic imaginable," said Twitter, "whether it's a unique bird's-eye view of the Shuttle launch as seen from an airplane window or cheers of support for soccer teams in this year's Champions League tournament."

During the first half of the year, however, a particularly large portion of tweets have been about teenage viral sensation and "Friday" singer Rebecca Black.

Britney Spears's album Femme Fatale has captured the ears and fingers of Tweeps, as has former Two and a Half Men star Charlie Sheen.

On a more serious note, Twitterers used their online clout to disseminate news articles and opinions about the AH1N1 virus, former Egyptian President Mubarak, and Libyan political leader Gadafi.

Tears and grief were shared around the world on the social networking site as Twitterers learned of the May 11 earthquake and tsunami that devastated Japan, leading the terms "#prayforjapan" and "Fukushima" become some of the most tweeted topics during the first half of the year.

Every day, as "the world writes the equivalent of a 10 million-page book in Tweets," Twitter is keeping tabs on the most popular topics around the globe.

The following terms are the most popular trending topics on Twitter for the first half of 2011, divided into the two most popular categories:

World events/News

  1. AH1N1 - Swine Flu
  2. Mubarak - former Egyptian President
  3. Easter - Christian holiday
  4. Cairo - capital of Egypt
  5. #prayforjapan - sentiment following the March earthquake and tsunami
  6. Chernobyl - site of nuclear disaster in 1986
  7. Libia/Libya - site of an ongoing civil war
  8. Fukushima - Japanese nuclear power plant
  9. William & Kate - Newly named Duke and Dutchess of Cambridge
  10. Gadafi - Libyan political leader

Pop culture

  1. Rebecca Black - pop singer
  2. Femme Fatale - newly-released Britney Spears album
  3. Charlie Sheen - actor
  4. #tigerblood - hashtag popularized by Charlie Sheen
  5. Nate Dogg - rapper
  6. Anderson Silva - Brazilian mixed martial artist
  7. Tom & Jerry - famous cartoon
  8. Mumford & Sons - British rock band
  9. Bieber alert - referring to artist Justin Bieber
  10. Queen Gaga - referring to artist Lady Gaga