According to this image of Kao - chan from the garden, around 350k users have signed up. If around 5% of people switched to Hankos, and so do not have a garden, then we have around 367k users. However, it seems to me that many of the active users are always the same, so I estimate there are less than 5000 users weekly.
If you're looking at the leaderboards, the top users are crazy people who spend a super ton of time studying so those barely change, especially the all time ones because the records are rarely beaten. And like ansair said the forums are not representative of the active user numbers at all. Where do you get that number from?
kind of rephrasing what's above but the forum is a really inaccurate way to represent the active users on renshuu because like 80% of the people who use renshuu use it for the sole purpose of studying/learning japanese--plenty of people don't even know the forum exists
Also the school year for a lot of american schools has started, so that might explain less activity But yeah, as everyone has said, don't trust the forums for an accurate estimate of active users.
But yeah, as everyone has said, don't trust the forums for an accurate estimate of active users.