This has come up before and I think it's more than we think. I know I get emails all the time from people who never post on the boards, referencing posts. In addition, there are people I see at shows who never post whom I've never met who say they "recognize" me from the boards.
I think there's probably a small amount of adult webmasters who don't have any board contact at all, but I do think it's a small amount.