So there are a lot of factors here that can influence the overall results.
Firstly, you are correct that there are a lot of people joining as I'm seeing a list daily of maybe 6ish people a day joining and the spam filter hasn't really been that active lately. In saying that the spam filter works on IPs and select information, like URLs and duplicate posts. Meaning that it may not trip until a not actually posts (or someone new accidentally uses a weird URL or double post in their first few messages).
This means that there is a chance that some of these accounts may be what we call silent bots, bots that don't activate until later on in an attempt to avoid the regular scans that forums have on new users.
However, of the ones that aren't bots... a portion of those have jumped into PM 1x1s. We can see this as each user has a Post count and a PM count that is kept private from the public's view, and some of these users have already made it into the hundreds.
Keep in mind though that, as mentioned above, the Online members count doesn't include Invisible users, nor will it really show people who have logged into an Alt account instead of their main. The count only shows a period of 15 minutes so unless a back to back log in is used by a person with an Alt, their count will be marked as 1 instead of 2.
Staying on topic with User counts and how it's a 15 minute internal... this is a choice by Mahz instead of Dailys as I believe it helps to show current activity rather than a daily summary. When I started this post it was sitting at 164, now it's risen to 172, but in an hours time it could drop down to 80. It's too much of a variation to make the theory that the forum is gaining or losing activity and even without these figures there's a lot happening in real life that may impact the overall activity. Halloween is on around this time, Thanksgiving is coming up, and I know Australia has all their exam periods starting soon too. This time of year does get a bit slow and last year's NaNoWriMo distracted a few people too.
Pretty much I wouldn't worry too much about the numbers and figures at this point in time. Yeah they're important but if you use one form of data to judge a forum's activity then you miss out on countless info that may be playing behind the scenes.
