Twitter users are now sending 200 million tweets per day, compared to 65 million tweets per day a year ago. In January 2009, users were sending 2 million tweets per day — so the latest figure really underlines just how much Twitter has grown over the last couple of years.

To put the latest figure into perspective, Twitter users are writing the equivalent of a 10 million-page book or 8,163 copies of “War and Peace” every day. Stacking this many copies of Leo Tolstoy’s epic novel would reach the height of about 1,470 feet, almost the same height of Taipei 101 in Taiwan — the second tallest building in the world. To read all of the tweets that are sent in a single day would take more than 31 years.

The news means that Twitter users are now sending 1.4 billion tweets per week, up from around 1 billion the last time that Twitter released official stats.

To celebrate the latest milestone and to mark the halfway point of the year, Twitter has announced details of the top 10 Trending Topics for the first half of 2011, divided by World Events/News and Pop Culture — the 2 most popular categories.

World Events/News

1. AH1N1 — Swine Flu.

2. Mubarak — the former president of Egypt.

3. Easter — a Christian holiday.

4. Cairo — the capital of Egypt.

5. #prayforjapan — sending thoughts to those in Japan, following the March earthquake and tsunami. On Wednesday, Twitter visualized tweets to and from Japan immediately following the earthquake in some cool videos.

6. Chernobyl — the site of a nuclear disaster in 1986.

7. Libia/Libya — the site of an ongoing civil war. NATO officials are using tweets to help determine strategic air strike targets in the country.

8. Fukushima — a Japanese nuclear power plant, which has a Twitter account.

9. William & Kate — the Duke and Dutchess of Cambridge, who married this year.

10. Gadafi — the leader of Libya.

Pop Culture

1. Rebecca Black — a pop singer who came under the spotlight for her “Friday” song and YouTube video.

2. “Femme Fatale” — the latest  Britney Spears album.

3. Charlie Sheen — an actor who set a Guinness World Record when he joined Twitter.

4. #tigerblood – hashtag made famous by Sheen.

5. Nate Dogg — a rapper.

6. Anderson Silva — a Brazilian mixed martial artist.

7. Tom & Jerry — a classic cartoon.

8. Mumford & Sons — a British band.

9. Bieber alert — a reference to musical artist Justin Bieber.

10. Queen Gaga — a reference to musical artist Lady Gaga, who has more than 11 million followers on Twitter.