Instagram photos are 100% public. Anyone in the world can see view them, whenever they want, without your permission. They're *published* on the World Wide Web.
Yet Instagram doesn't feel like publishing, and I don't think most people realize just how public their content is. Do my college friends know that their drunk Santacon photos are accessible to anyone? I don't think so.
Why is that? Does the context of your mobile device make you feel safer because it's more intimate?
To preempt a specific rebuttal, I don't buy the argument that "photos are easier than words so we don't care" because we freak out about Facebook's privacy settings (largely because of the photos we keep on there).