Yup, we get that question a lot here. It's annoying as hell when people don't respond. Part of the problem, in my mind, is that people tend toward these really large parties so guests figure you don't really care how many people come since you are prepared for a crowd anyway. We never did big parties at all, but a lot of people do. I think it's an expensive free-for-all that's overwhelming for kids, but not everyone agrees. I also think that the evites (while efficient) take away a level of personalization, so people feel the replies can be impersonal too. I understand about invitations at the bottom of the backpack, and every preschool and school I've worked at has asked parents not to give this job to teachers for a variety of reasons. So taking the extra time to find out the addresses is one way to make it more personal and hope for replies - but I know people get disappointed that way too. (I don't know anything about ECHOage vs. other sites - is there any possibility that they're getting mixed in with spam sites or sent to some people's junk folder?)
You have 2 choices, I think.
1) Follow up with a phone call. You may or may not get answers. Don't reprimand about how busy you are or how you have 25 goody bags to make.
2) Don't call, and be shocked when someone shows up. Look surprised. Stammer a bit. Then say, "Oh, my! Well, what a surprise. I had no idea you were coming... Did you send your reply to the wrong email address? Let's see...Well, let me see if I can squish another folding chair in. The kids are little so I'm sure your Billy will fit." They probably won't do it again. Or maybe they will. Some are clueless and will blame you. But you'll have made your point in a gracious way. However, that means you have to be prepared with extra food and supplies, and avoiding that was the point of the RSVPs to begin with.