Royal Pingdom reports on findings that Twitter saw its most robust month in January based on number of Tweets.  The internet monitoring firm tracked 1.2 billion Tweets for the month, accounting for feeds sent on Twitter and third-party applications.  Royal Pingdom says that represents nearly 40 million Tweets per day, and a 16-fold growth in usage of the service over the same month last year.  The firm says its findings gel with a statement from Twitter CEO Evan Williams, who has announced that Jan. 12 was the highest usage day in the service’s history.  Royal Pingdom also found an average 17 percent month-by-month growth in Twitter usage between November 2009 and January 2010.  The firm’s site provides a brief explanation of methodology in how it tracks Twitter.  Read more at Royal Pingdom.