Over 5 billion tweets have been posted

"The microblogging site only reached its one billionth Tweet last November, and was only up to 1.6 billion in April. The remaining 3.4 billion Tweets have been sent in just six months, suggesting massive growth in the use of the site.
There is some debate over whether the @robinsloan Tweet – which he is calling the “pentagigatweet” – is really the five billionth.
The number of a Tweet is displayed in its web address or URL. @robinsloan’s Tweet is at the URL twitter.com/robinsloan/status/5000000000 , or five billion.
However, Twitter has changed its numbering system on more than one occasion. It is not clear that Tweet 5,000,000,000 would have the exact URL number 5000000000.
What is definite, however, is that the five billion mark has been passed.
A third-party site called GigaTweet has been counting all Tweets for some time now. Its count at the time of writing had just passed 5,018,584,000, although it is going up by around 300 a second."
Source: Daily Telegraph, 20th October 2009