I think it depends on the forums.
The more laid back and generic the forum is, the greater the ratio of contributors I would theorize.
That is because the interaction is the purpose of the forum more than information.
But in a very specialized forum like stackoverflow, information is king and not only are you discouraged from posting if you don't have anything meaningful to add, but you are also strongly encouraged to peruse existing threads and look for your answer BEFORE you ask a question to avoid redundancy.
In such an environment, the ratio of contributors is unavoidably tiny, because maybe 1000 people will have the same question in a year, but only 1 person will ask it the first time and maybe an handful of people will answer it.
Actually, this makes me think: This forum might be more usable to a lot of people if it was divided along usability lines.
The Utopia forum is a good example of this:
http://forums.utopia-game.com/forumdisplay.php?1527-Utopia I guess the game specific forums are a start, but there should probably be a section dedicated to announcements and one dedicated to inquiries about the service GOG provides (as opposed to specific games).