You ask a question about "level of attendance" that always generates some good discussion.
First, the simple thing to do is to use ratings, giving 1 point per post, and using "count of ratings" to contribute towards a students grade. Right? Simple?
Well, not so. What will you define as a perfect ("A") score? 10 posts, 100 posts, 1000 posts? Also, once students catch on to this, they might start making many simple posts. For example, instead of providing a paragraph of ideas, they might instead provide sentences of ideas. One ends up with essentially the same problem when teachers try to use "number of words" as a criteria for grading papers... students can easily increase the number of words without any more intellectual content.
So consider what you really want from discussions, and try to configure your forums to encourage what you want. For example, you can set up a forum using ratings, where you expect a minimum of 5 quality posts, and then rating each post on a 1-4 scale based upon quality. If a student makes 5 excellent posts, they earn 20 rating points, which can be a perfect 100% (an "A+). The burden is on the instructor to read every post, and have some form of idea of what a "1" is versus a "2", versus a "3", etc. If a student makes 5 posts, let's say: 1 excellent, 3 good, and 1 fair, they have earned 15 points, and the student can decide if they want to make more posts in order to reach the 20 maximum points.
Well, this is one idea. It attempts to encourage "quality" instead of "quantity." Maybe others will suggest other ideas.