TimmyMagic - 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.