You only upload 1 photo.
True, someone can dump a lot of photos one at a time, but I think that's a bit of an instagram faux pas. It tends to be that if someone is on vacation (for instance) they don't dump all their photos on instagram, they choose their one best photo, crop it, add a filter to it, and try to make it great. You don't create an album like you do on facebook. This makes the quality of what you see in your feed a lot higher than what you would see on facebook. I've been told by a few heavy users of instagram that this is what they love about it.