As far as looking at whether the majority will snack on a slice of pizza, I would say its hard to bring proof from what we do ourselves. I know that I would find pizza good for snacking if not for the fact that I need to wash and bentch. A posteriori, it is suitable only at mealtimes.
Can we judge based on what non jews do? If so, I still don’t know if by majority it is a snack, but I know that I frequently see people eating pizza on the go. It could very well be true that one slice is considered a snack to most people in the US.