Since this is my day to post X vs. Y entries, Twitter has hit 50M tweets per day. According to About.com as of 2008 there were 183 billion email messages being sent each day by 1.3 billion people of which around 70% is spam or viruses (which I found surprisingly low; but then I suspect Twitter’s signal/noise ratio is even worse and there are no good filters). Twitter has been growing at a rate of 400% every six months, so at its current rate of expansion it should hit 183 billion tweets in three years, give or take. (Email growth is considerably slower—it averaged 14.6% from 2001 to 2007.)
Of course, in three years we’ll all be using Google Wave on our Windows Tablet 7 Series Tablets.