It’s Twitter’s sixth birthday, and to celebrate the company has posted some new stats over on the official Twitter blog.

They’re big, too: Twitter now has “more than 140 million active users” sending a heady 340 million tweets each and every day.

Or, in another words, over a billion tweets every 72 hours.

Take that in for a second. Wow.

Like Facebook, Twitter defines an active user as someone who logs on at least once per month. Yep, that doesn’t strike me as particularly active, either, but it’s become the industry standard for the term now.

Back in January I speculated that Twitter was on track for 138 million active users by March and it looks like that analysis was pretty much spot-on. By my calculations, Twitter should see 250 million active users by the end of 2012, and some 900 million registered profiles overall.

This is a big day. And for Twitter, I have a feeling it’s just a small part of some even bigger things to come.

(Source: Twitter blog.)