Several kindly peeps let me know about this tool awhile back; thought I'd share the good word since I've been using it for awhile: Flickr Group Trackr from Nitens. Here's its chart for the biggest Second Life group:
Click through for more detailed stats. It doesn't start charting until you enter a group, so you might as well get groups of interest entered now, then you can come back later and peek on progress. Nifty.
