Daily Tweets levels can be found here - http://tweetrush.com/
Compete metrics here - http://siteanalytics.compete.com/twitter.com/?metric=uv
State Of The Twittersphwere (Q4, 2008) here: http://blog.hubspot.com/blog/tabid/6307/bid/4439/State-of-the-Twittersphere-Q4-2008-Report.aspx
Hitwise says web visits have increased 8x in the last year, albeit from a minuscule base.
Compete shows about 900,000 U.S. monthly website visitors in 2008
Comscore puts the worldwide number at 1.3 million unique monthly visitors in March 2008 - None of that data is particularly useful, since so much of the action on Twitter occurs via mobile phones, instant messaging and desktop clients like Alert Thingy, MySocial24×7 and Twhirl. Many of Twitter’s most active users rarely visit the website.
The key measure of Twitter usage is total users, total active users and total messages sent. And according to a source close to the company, these are the current Twitter usage stats:
Total Users: 1+ million
Total Active Users: 200,000 per week
Total Twitter Messages: 3 million/day
Note: It’s not clear what the 1 million total users refers to, since there are many more registered users (see comments below). It may be users who’ve left at least one message, or an active user number that is over a longer period than the weekly number (monthly, for example). Those stats have roughly doubled just since January, when Twitter had just 100,000 active weekly users. Previously it took nine months to double in size - In April 2007 the service had 50,000 active weekly users.
What’s most interesting is the rabid Twitter usage by active users - they send an average of 15 Twitter message per day.
Companies on Twitter (taken from article - http://www.churchofthecustomer.com/blog/2008/03/today-ben-marti.html)
* Promotions. Southwest Airlines uses Twitter to let customers know about deals. Today I got a tweet from them explaining that if I book a ticket using PayPal, I'd get $50 back in PayPal credit. Southwest also tweets press happenings and pointers to blog posts, as they did recently about their plane inspection issues.
* Listening and responding to customers. A few days ago, I tweeted about how Starbucks and Dell are using Salesforce.com's Ideas product for their Ideastorm and MyStarbucksIdea.com, respectively. But I had inadvertently linked to the wrong page on Salesforce.com's website. In less than 30 minutes, someone at Salesforce saw my mistake and tweeted the correct URL to me. I posted the correction back to Twitter along with a compliment about Salesforce listening in on the Tweetersphere.
* Employees as ambassadors. Dell leads the world with employee representation on Twitter. Check out the conversations that these Dell employees are having on Twitter: LionelatDell, RichardatDell, JohnatDell, APaxtonatDell, KellyatDell. Unlike many Twitterites, I met most of the Dell people in person before following them on Twitter. Because I had a chance to cement a relationship in person, I'm a big fan of the Dell people I follow on Twitter.
* Track your fuel consumption: http://www.socialtimes.com/2008/05/track-your-mileage-with-fuelfrog/
Jet Blue on Twitter - http://www.socializedpr.com/jetblue-engages-in-real-conversation-on-twitter/
Twitter Update (April 2009)
Twitter (Twitter) is now growing at a mind-boggling 2,565 percent. In total, it reached more than 13 million people in the US during the month – and that’s just on its website (i.e. – not counting clients like TweetDeck (TweetDeck) or Seesmic Desktop (Seesmic Desktop)). Next month will likely see this total continue to swell, thanks to huge mainstream media events like Ashton Kutcher versus CNN and Oprah jumping onto Twitter.
James Norris Cited in Mashable