Silencertalk.com. It's pretty quiet, but lots of good info.
Form1suppressorboards.net is a good place to learn about them, but it's about Form 1 stuff, so more on design & theory, not much discussion of production cans.
AR15.com silencer subforum is probably the most active
And as Demi-human said, here. There are a few of us here who are heavily involved with cans. >95% of my work is suppressor development, manufacturing, modification & repair. The remainder is mostly making hosts suitable/more suitable for suppressed use.