The first point that has stuck with me during this thing is the fact that when you do comment, it should be about something you you are passionate about and that you genuinely care about. I think readers will be able to sense whether or not your tone is natural or forced. So, although commenting is important, it is more important to care about what you are saying. I totally agree with this...the best public speaking advice anyone ever gave me was to focus and care about what I was saying instead of worrying about what people think. I think this somewhat transfers into virtual "speaking."
I also like the point made about inviting comments. I know I am a lot more likely to comment when someone has made a direct invitation, or has posed a question. These types of posts just lend themselves for more interaction.