I don't use PM's very often, but there is almost 30 pages of messages in my inbox. Usually I communicate with people about my own website and stuff on - they requesting to add something to it or just giving suggestions how to improve it. Also, I use PM when I want to hire someone for job/task or when I want to be hired. Bitcointalk football prediction pools is another PM topic. Also, when I was in old Yobit campaign, discussions with other participants about campaign related issues was often thing
I noticed one trend in Service sevtion and Marketplace - when OP asks to contact him via PM, but people are still replying to topic. And we all know reason why they are doing it