User blog comment:Sarah Manley/2 million users on Wikia/@comment-8-20100419185036

The question of how many are active is trickier than you'd think... partly because log-ins are global and activity is local. So Uber looked at the number of people who logged in so far this year. Some of those may have gone since logging in, and some may not be counted because their session has been preserved (although over 4 months that should be few)

And the answer is 310,253 users or around 15% of accounts have logged in this year.

Anons are even harder to count, so this doesn't include them.