There are 5-13 PF groups up at any given time. It's 8:30AM PST and there are 5 right now. In my experience, around 4PM-11PM PST is when you see the most activity. I don't know what that is compared to what you are used to, but if you do not have an active LS or FC then it can seem pretty dead. If you get with a good LS or FC then I'm sure they have groups that they don't advertise in PF.

I find myself using Duty Finder much more often than PF which can be frustrating on hard content.