In March, 2010 there were 50 million tweets per day.
In March, 2011 there were 140 million tweets per day.
In May, 2011 there were 155 million tweets per day.
Yesterday, apparently, there were 350 billion tweets per day.
350 million tweets/day would have been an astonishing 2.25x growth in just two months, where previously tweet volume has been increasing by 3x per year. 350 billion tweets/day is an unbelievable 2258x growth in just two months.
Quite unbelievable. In fact, I don't believe it.
350 billion tweets per day means about 4 million tweets per second. With metadata, each tweet is about 2500 bytes uncompressed. In May 2011 the Tweet firehose was still sent uncompressed, as not all consumers were ready for compression. 4 million tweets per second at 2500 bytes each works out to 80 Gigabits per second. Though its possible to build networks that fast, I'll assert without proof that it is not possible to build them in two months. Even assuming good compression is now used to get it down to ~200 bytes/tweet, that still works out to an average of 6.4 Gigabits per second. Peak tweet volumes are about 4x average, which means the peak would be 25 Gigabits per second. 25 Gigabits per second is a lot for modern servers to handle.
I think TwitterEng meant to say 350 million tweets per second. Thats still a breaktaking growth in the volume of data in just two months, and Twitter should be congratulated for operating the service so smoothly in the face of that growth.
Update: Daniel White and Atul Arora both noted that yesterday's tweet claimed 350 billion tweets delivered per day, where previous announcements have only discussed tweets per day. That probably means 350 billion recipients per day, or the number of tweets times the average fanout.
Update 2: In an interview on July 19, 2011 Twitter CEO Dick Costolo said 1 billion tweets are sent every 5 days, or 200 million tweets per day. This is more in line with previous growth rates.