I kind of agree with Eternad. I guess visibility shouldn't immediately be equated with the number of posts per week or the number of participants. There are other factors for a post, and therefore the signature, to be more visible than another.

BestChange, for example, implements a rule that only 2-3 posts made on threads beyond 5 pages are to be counted. This is because mega threads are usually full of spam and posts beyond the 5^th page are more easily missed than those made in the first 2 pages. Also, posts that are already buried deep in mega threads are normally less interesting because they are most probably just repetition of earlier posts, and posters probably didn't even read everything before joining the discussion.

Another factor perhaps is the level of influence of the user wearing it. If theymos, for example, wears a signature, it becomes very visible to everybody even if he only makes a single post per week.

Perhaps a more reliable measurement of visibility is the number of visits made to their websites through signatures, or perhaps the number of users referred by signature links.