Twitter CEO Dick Costolo is holding an informal business meeting at Twitter today. TNW is there and Costolo is sharing some interesting statistics. The chief among those is that Twitter currently has 100 million users a month, half of which log on every day and 55% of which log onto Twitter on a mobile device. Twitter is currently seeing 5 billion Tweets every 5 days.

Since January of 2011, this number is up 82% and their active users are growing faster this year than last. The growth isn’t just stateside either, as the worldwide grown of twitter is neck-and-neck. Twitter is on track to add as many active users in the next 4 months than they added in 2006, ’07, ’08 and ’09 combined, 26 million.

Costolo also said that Twitter.com  has more than 400 million monthly unique visitors, according to Google Analytics. That number is up from 250 million in Jan, ’11. “40% of our monthly active users do not tweet, ” said Costolo, “This means that they have not tweeted in the last month”

Twitter defines active users as “people who have logged in during the last 30 days via Twitter’s own apps or any third-party apps.

55% of Twitter’s active users are active on mobile., which is an increase of 40% quarter over quarter. Twitter is also getting 400 million unique visitors a month total, this points to a huge number of users that visit Twitter just for information without participating. “The 400 million monthly uniques number shows that people are getting value out of Twitter without logging in,” said Costolo.

Costolo also discussed how Twitter uses an experimentation model to roll out new features, rolling them out to 2%, then 5% then 100% if they ‘work’. Eventually, Twitter wants to be on over 2 billion devices.

Costolo didn’t address Twitter spam, an ongoing issue for heavy users, aside from saying that Twitter is ‘doing a much better job with spam’ and that they “have a bigger team assigned to that.”

Of the 40% of inactive Twitter users on Twitter, Costolo says that Twitter is attempting to move many of them as possible into simple publishing actions, getting them to begin actively participating in Twitter, rather than just consuming. Costolo described the process, saying “you come in, find things you’re interested in, get a timeline, and start engaging that way. Maybe after a while, after you’re following a bunch of accounts, you start to re-tweet. and then you start to reply to things after re-tweeting.”

Twitter also shared with us some of the more interesting stats about those 100M active users:

  • Every team in the NFL is on Twitter and more than 50% of NFL players
  • 75% of NBA players
  • 82% of the US Congress and 85% of US Senators
  • 87% of the Billboard Top 100 Musicians of 2010
  • 93% of Food Network chefs
  • 100% of the top 50 Nielsen-rated TV shows

Twitter has experienced massive growth this year, bolstered by popular public events like the Womens World cup as well as unrest like the London Riots and disasters like the earthquakes and Tsunami in Japan. Users of the service are relying on it more and more to provide them with up-to-the-minute information about what’s going on and to keep in touch with loved ones during emergencies.