Nearly half a million twitter accounts are created each day - Official twitter statistics, March 2011

"Today, on every measure of growth and engagement, Twitter is growing at a record pace. Here are some numbers:
#tweets
3 years, 2 months and 1 day. The time it took from the first Tweet to the billionth Tweet.
1 week. The time it now takes for users to send a billion Tweets.
50 million. The average number of Tweets people sent per day, one year ago.
140 million. The average number of Tweets people sent per day, in the last month.
177 million. Tweets sent on March 11, 2011.
456. Tweets per second (TPS) when Michael Jackson died on June 25, 2009 (a record at that time).
6,939. Current TPS record, set 4 seconds after midnight in Japan on New Year’s Day.
#accounts
572,000. Number of new accounts created on March 12, 2011.
460,000. Average number of new accounts per day over the last month.
182%. Increase in number of mobile users over the past year.
#employees
8. 29. 130. 350. 400. Number of Twitter employees in Jan 2008, Jan 2009, Jan 2010, Jan 2011 and today."
Source:  Blog post by twitter, 14th March 2011