Member lists are not purged on some sites and are usually bloated with accounts that never finished the activation or if they did finish never used it.
I purge the registered accounts on my forum every few months, anyone with 0 post account with no activity in six months is removed. Those that have been purged make up 50% total that have registered. If you threw in the spammers that have registered and posted its probably goes up to 60%
Looking over some stats of those remaining on the list active members is probably in the 20% range if you wanted to consider the beginning of May as being active.
Msg#: 4337660 posted 8:03 am on Jul 15, 2011 (gmt 0)
Just to add you're not going to have a norm. Active participants as a total will increase but the percentage is going to drop. If I had to guess the majority of people that use my forum usually end up in the 10 to 20 post count range and then leave after getting the information they wanted.
Msg#: 4337660 posted 9:13 am on Sep 4, 2011 (gmt 0)
90% of users will be lurkers. 9% will contribute from time to time. The remaining 1% will participate a lot and contribute for most activity. This is known as the 90-9-1 rule.
Of course we can debate the percentages and they will vary, but the point is that active users will generally be a very low percentage of total users.
@thecoalman - Why would you deleted a user with 0 posts after six months? I have a couple of members of my forum who were lurkers for many months and are now among the most active. It's unlikely someone will become active after 6 months of inactivity, but I'm not sure why you'd delete them.
All that matters to me in a forum is that the content keeps increasing (people are participating) and the number of visitors (registered or "lurkers") each day increases over time. Just as important is that threads people start are replied to, especially if someone starts a discussion thread with a question, is the question answered. Each person who keeps a forum online is so different in how they manage aspects such as pruning inactive visitors and such makes active users as a % of registered users a useless number/statistic. Even the number of people online is dependent on the forum topic/focus. If it's a business advice forum it may not have many visitors online during weekends, for example, but during week days during business hours (depending upon how "local" the forum is to a geographic audience) the forum may have 10x or more visitors online than on a weekend day. On top of that, the majority of visitors to a forum do not register at all. They lurk, get the information they're interested in and move on. And hopefully they come back for more information.