With all the tweets that are being sent out by the web, mobile devices, toilets, airplanes, and from outer space you would figure that at least a million were being sent out per day. Pingdom quantifies that exactly 27.3 million tweets are being sent out per day as of right now.
The sample dates for that average is between October 21 and November 11. There are fluctuations of between 567,000 and 1.8 million every hour depending on major events taking place. When extrapolating that average, it comes out to over 10 billion tweets per year. While this is a large number, Yahoo! members send out 100 billion e-mails per month according to TechCrunch.
In terms of communication preference, Twitter will not be able to replace e-mail but it can come darn close.