The most I saw was about 10 in there. We added it in the first instance because we created a similar system for the CTRA admins, so that we could work more effectively together on reports. It was easily portable, and seemed pretty good code, so we stuck it on MyCTRA. However, for some reason we have yet to properly work out, the processor load on the server was massive. If one person joined chat, one of the server's processors would immediately jump up in usage by 20%. Basically we couldn't allow MyCTRA Chat to get popular until we figured out the issue.. we had to drop it.
I'd thought about a PM system, but as you point out there's already that option here on this forum. We have in-server team messaging across servers and AFAIK, that's never been much used. I don't think LFSers are lacking in the ability to communicate that we at CTRA can really help them with, at the end of the day.