This is a very good analysis and I think one of the things that might help get people over to comms is to incentivize posting there - like, kick a fandom community off with a month-long event where up to once per person per day each post gets the poster a raffle entry, your first daily comment on someone else's post is too, etc. And then post about that event on Tumblr, Twitter, etc.

I had planned to do something like that but haven't had the executive function to organize it, unfortunately. But I'm submitting the idea in case someone else does.