Also, be sure to have this patch in place though: http://status.net/open-source/issues/3540#comment-43423 , it does help quite a bit as far as actually sending out notices to everyone subscribed. After having this in place, if one of my groups would have quite a bit of activity, I could easily see anywhere from 10GB to 50GB of data transfer a day from my server. Of course, this is including everything else I have running. ;)