and less high quality participation (but then, overall volume was smaller as well).

Don't underestimate the impact of voting mechanisms in their ability to screen out poorer-quality discussion. (As someone who is frequently downvoted on LW, I can state from personal experience that while this is often done persistently on the basis of individual sentiment rather than in terms of contribution to the dialogue, the impact is relatively equivalent either way: by seeing the opinions of others to the comments of others, we learn how to better sculpt our own comments.)