At today's Twitter news event, co-founder Evan WIlliams revealed a number of impressive statistics about the microblogging network. For example, Williams said Twitter is seeing on average 90 million Tweets per day. As you can see from the graph we've embedded, this is a significant increase from even a few months ago. We're assuming this stat includes Tweets from both Twitter.com and clients. Williams also said that since the beginning of the year Twitter users on mobile devices are up 250%, with 16% of new users starting to use the service on mobile devices. And there are on average 370,000 new sign-ups per day overall.