Twitter announced today that its micro-blogging service experiences 50 million tweets per day (TPD), an increase of 1,400 percent from 35 million TPD in 2009. In an effort to track the company’s growth over the past few years, the Twitter analytics team has posted a TPD report on the company blog, revealing that people were tweeting only 5,000 TPD back in 2007. According the blog post, the 50 million Tweets per day milestone does not include tweets from accounts identified as spam.