Twitter is counting down to the end of 2011 with a five-part Year In Review website: a snapshot of the year that lists the trends and happenings in, as well as notable joinings to, the world of Twitter.

It began on December 1 with stories made possible through the microblogging platform such as Shohaib Athar inadvertently live-tweeting the raid on the Osama bin Laden compound in Pakistan.

December 2 brought a list of notable joinings to the Twittersphere, from tennis star Roger Federer, to the president of South Africa, to the US Secret Service.

And December 5 saw the revealing of the hottest topics to be tweeted about over the past year. As well as lists of the most discussed entertainment, sport, technology and world event topics, Twitter revealed 2011’s top eight hashtags:

1. #egypt – Social media played a notable role in the pro-democracy protests and uprisings in Egypt earlier this year, with the #egypt hashtag being attached to rallying cries as well as calls of support from all over the world.
2. #tigerblood – Despite zoology’s claims to the contrary, Charlie Sheen’s assertion that he possesses tiger blood, giving him superhuman powers, caught on.
3. #threewordstoliveby – In which Twitter users list the three words they hold to be very important in their lives.
4. #idontunderstandwhy – In which Twitter users list something they don’t understand. Obviously. Often about the opposite sex.
5. #japan – The massive participation in social media in the aftermath of the earthquake, tsunami and nuclear disaster showed Twitter’s potential in emergency situations, as well as being used to deliver messages of support from around the world.
6. #improudtosay – In a similar vein to number three, users tweet something they’re proud of.
7. #superbowl – The Superbowl was popular on Twitter. Enough said.
8. #jan25 – January 25 was the day Wael Ghonim became a symbol for the Egyptian democracy movement (see 1) after being detained and presciently predicting the role of the internet in the movement and in activism in general.

Part four includes stats on peaks seen in the number of tweets per second over the year, climaxing at 8868 tweets per second during the MTV Video Music Awards on August 28.

The fifth and final part of Twitter’s Year In Review is yet to be revealed…