The last time Digg officially announced how many registered users it has was back in March when it passed the one-million mark. Now, programming sleuth John Graham-Cunning extracted registration dates from Digg user profile pages to come up with an estimate of 2.7 million registered Digg users. Some of the key inflection points came in June, 2006 when Digg expanded beyond technology to cover world news and entertainment, and in December, 2006 when it added video links. The makeover in August, 2007 seems to have helped as well. At this rate, Digg should surpass the three-million user mark by March, 2007.

Here is Graham-Cunning’s unofficial chart of Digg registered user growth:

These are just registered users. And we don’t know how many are active, but you need to be a registered user to participate in the Digg community (i.e. to submit stories and vote them up the page). Digg’s overall traffic is higher. According to comScore, it peaked in October, 2007 at 6.3 million unique visitors in the U.S., and came down a little to about 6 million in December (this dip might be seasonal due to college students being on break for the holidays). Worldwide traffic in December was about twice that number in December.