A Post Count Monkey (PCM for short) is a member of the forum who tries to collect significant post numbers, for example the 10000th post. They do this by starting a new thread with a title of 10000 (or whatever the number they're claiming is).
There was a time where about 90% of the threads on the forum were PCMs claiming numbers, but fortunately they now restrain themselves somewhat (they seem to only be going for every 1000 threads). PCMs can be easily identified by their excited-to-the-point-of-wetting-themselves posts when a biggie is approaching. They should be considered dangerous, and should not be approached for fear of catching the disease.