The homepage for Twitter.

 

Twitter users send out 200 million tweets each day, according to 2011 statsfrom the social networking service.

During the first half of the year, 1 billion updates hit the site every five days.

For some perspective, Twitter notes on its blog that "the world writes the equivalent of a 10 million-page book in Tweets or 8,163 copies of Leo Tolstoy's War and Peace."

What are users tweeting about? Here are this year's top trending topics, according to Twitter, grouped by World Events/News and Pop Culture.

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 duchess of Cambridge

10. Gadhafi - 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