I have never followed up either. Sometimes I wish I was brave enough to do it. My children are 22, 13, and 12 so I have had many birthday parties with a large amount of people that don't seem to know what RSVP means. I have found that MOST of the time if they don't call they are not coming. That is not always the case so I usually make enough party bags for all the kids that I know are coming and the ones I didn't hear from. Now that my youngest two are older they just have a couple kids over to spend the night for their birthdays. It is much easier.