EDIT: Dan Huard just commented on my Digg submission saying that this number is not correct. My apologies.
Although I was unable to get an official count from Digg HQ, I used my tried and true method of trial and error to find out exactly how many users there were registered at Digg. Basically that consists of blocking user numbers hoping there is a corresponding user until I narrow it down to the exact number.
For those wondering who user 700,000 is, meet meihuanew who registered at about 1:58 pst.