With half of 2011 gone, Twitter has announced a new milestone: its users are now sending over 200 million tweets per day. In other words, 1 billion tweets are sent every Monday through Friday. In January 2009, Twitter users sent 2 million tweets per day. One year ago, that number was 65 million per day. Twitter's growth is accelerating, at least when it comes to tweets.

To put the 200 million number into perspective, Twitter says its users write the equivalent of a 10 million-page book in tweets or 8,163 copies of Leo Tolstoy's War and Peace, every day. Reading this much text would take more than 31 years. To celebrate the milestone, Twitter has released the top Trending Topics for the first half of 2011:

  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

Last month, Twitter started to roll out its own photo sharing feature and pushed out a new version of search for the site. Two months ago, it unveiled the Follow button, which lets you instantly follow Twitter accounts directly from the websites you visit. Three months ago, the company started to roll out a new mobile website. Four months ago, the company launched a new homepage.

I do not have Twitter for personal use, but I do use it for work. You can follow @TechSpot for news from the site or follow me directly: @EmilProtalinski.