The accurate number of both registered and unregistered users in all of Stack Exchange network at the moment of writing the answer is: 5,178,736.
var totalUsers = 0;
totalUsers += parseInt($(this).find(".number").find("span").first().attr("title").replace(/\,/g, ''));
Note that the per site Users page (e.g. Stack Overflow users) show only registered users so it's also a way to calculate amount of unregistered users per site. For example on Stack Overflow, there are 75204 pages of users, 36 in each page resulting in about 2707344 registered users. On the sites list it says "2,990,649 registered and unregistered users" so it means 283,305 unregistered user accounts.