According to my calculations: around 2.7m registered users.
I obtained this number by finding random Digg users and extracting their user id. The user id is in a hidden HTML form input field on each Digg user's page. The Digg user page also gives their date of registration. Using this I was able to plot every month from December 2004 (when Kevin Rose registered) up to this month.
Here's a picture:
Of course, the user id might be being generated in some other way, but I suspect that it's an auto_increment field in their database. To validate my data I checked the Digg blog and in March 2007 Kevin Rose wrote that Digg had 1 million users. This ties directly with my data, and hence I think I'm correct.
If this data is correct then looking at the last 6 months Digg is growing at a rate of around 110,000 users per month.
Now that doesn't say how many are active, but it's interesting data nonetheless.
1. The inflection point at June 2006 corresponds to the launch of Digg v3 which marks the point that Digg stopped being just about technology and added categories like World and Business and Entertainment.
2. Kevin Rose was on the cover of Business Week in August 2006.
3. Another calibration point is this post listing the number of Digg users as around 700,000 in December 2006. That also matches my data.
4. There's a big jump of users in December 2006 when Digg added multimedia features like videos.
5. The May 2007 jump corresponds to the AACS key "controversy".
6. Yet more confirmation that these figures are correct. TechCrunch reports that Digg had 200,000 users in March 2006.
7. "Kurt" pointed out that the Digg API gives a user count of 2,211,964. That's interesting because it gives us a way to estimate the number of spammer/abuser accounts banned by Digg: about 500k or around 19% or registered users.