Steven, we are running at 10-15 new members per month here - it takes time to build up a user base but it's sort of exponential. As more members join and more people visit, the site gets noticed by the search engines and appears further up the search screen which attracts more interest etc.
It's a double-edged sword sometimes as John has found in the past. If we attract the attention of the spammers, they can cause mayhem by using our site to post spam email and/or porn. That's one of the reasons why he switched to the SMF software. It's also the reason why we use an email registration system on sohc4.net - we were getting 40-50 registrations per day , mostly from .ru domains (Russia!). The only way to sort that is to have the mods approve each new registration - about 300 per month (Bob Wessner does most of them!)
Let's hope it doesn't come to that over here.
