So, as a self-proclaimed "community ghost" There are a few reasons as to why I tend not to discuss things.
1) it can be shyness-- sometimes I don't feel comfortable engaging in a discussion as I don't always know what is going on or what to offer.
2) It's already been said. If I see something that I agree with, typically in discussion threads such as this one, I'll just +floof replies that I agree with and let others with new perspectives add to the conversation. In my opinion, I don't need to be adding, effectively anything, to a conversation that could otherwise benefit from well-worded responses.
3) Forums games aren't really my thing? A lot of what gets responded to, on this forum especially, are the forum games. I've responded to a few over the years, but they're not something I find actively engaging myself. Sometimes I browse the games to see what people are saying or doing-- because the ideas are cute-- but I tend not to participate in them.
4) I think-- I'm always online? LOL As crazy as that is I always have this tab open to check back on and that's partly because I run RP groups on here. If someone responded to a forum ad, I would want to be able to respond quickly.
To sum it all up-- Any community I've been in I haven't engaged a ton in the forum aspect of it. Usually, I play a game to enjoy the game, and since the forum is such a small part of the game (not really in the case of FH I suppose), I tend not to even remember it's there. If it weren't for the play-game tab I probably wouldn't be on this forum much at all.
I will say using this forum for maps, meshes, member tutorials, presets, markings, items-- VERY helpful. Not to mention the history of bugs/issues reported on this forum usually means you don't have to start a new one to fix the game if it breaks. I support forum use 100%! I just think some of us would rather browse then add to it.