I know exactly how many of my friends don't use Facebook, because I specifically remember which ones they are, so I can message them separately.

It's inconvenient for me, but I'm happy enough to accomodate them.

I read your post as you were some sort of event planner or something, not that you were using it in a personal group of friends. Please forgive the misunderstanding. I see a lot of groups/businesses that use facebook exclusively to communicate and organize events (like the local paintball field) and they are missing out on some people (I have no idea how to quantify how many). I'm sure they're reaching more people now (using facebook) than they were using whatever old method they were using.

Yeah, I'm in the same boat, but it's easy for me because I'm thoroughly unlikable and have few friends anyway.