The problem of PM is, their location of funtionality design is a little hidden, normally are users who so involved use it. You can see that only will be discovered inside of the user menu (Specially from mobile view)
My suggestion just go to offer a so clear communication ability from FLAT....you register in my forum, when you use START TOPIC , you just can go with that private topics with your friends, or just casual topic that you want to talk with some members.
START TOPIC is the engagement point in whole IPS environment. PM not, at least in this moment...maybe if IPS want to bring PM functionality more front, then it will be replace what I am suggesting