By way of comparison comp.lang.perl.misc has had approximately 550,000 posts since May 1995 based on the data here.
These results aren't exact. For instance only about 90% of the nodes on Perlmonks are actual questions or answers.The rest are administration or home nodes. Also I don't know if the figures reported by Google Groups for clpm are accurate.
Nevertheless, the data is interesting. It shows that clpm's peak was in July 1999 (about the time that I started hanging out there but I doubt that has anything to do with its decline). It also shows that currently Perlmonks is getting about 8,500 posts a month in comparison to 2,500 on clpm.