I noticed that SuperPoster label when I was still quite new here. I wondered why I was a SuperPoster with my 5 or 6 posts while others with hundreds weren't.

As far as I can tell, it depends on what the Statistics tab of your profile shows. For instance, Snowleopard56's says:

Total Posts: 13
Posts Per Day: 4.60

Everyone I've seen with more than 2.0 posts per day seems to be a SuperPoster, but nobody with 1.9 or fewer. I'm pretty sure I can guess what the magic number is!