20 billion tweets have been posted

"There have now been more than 20 billion tweets since Twitter (Twitter)’s inception, according to tracking service GigaTweet.
The milestone comes just two months after the service hit 15 billion tweets and about five months since it reached 10 billion, indicating that activity levels on the microblogging service continue to accelerate.
While GigaTweet’s count is unofficial, Twitter indicated earlier this year that they’re seeing more than 50 million tweets per day. With the company recently breaking its own activity records during the World Cup (World Cup), we imagine the numbers are fairly accurate."
Source:  Data from Gigatweet, reported by Mashable, 31st July 2010