Those three categories are comprehesive, because when I hit your site I'm always looking for something. Sometimes I don't know what it is. Sometimes I'm looking to satisfy my curiosity. Sometimes, all I'm looking for is something, anything, to distract me from what I ought to be doing. But I'm looking for something.
If your visitors can't find what they're looking for, you can adjust your content to better fit your existing audience, or work towards directing a more appropriate audience to your pages. You should probably be doing a bit of both.