Would you respond to be polite? Or just smile and leave it at that?

You want to respond so that you don't come off as being stuck up or "too good" to answer your "fans." But at the same time, you don't want to start something that could get creepy. A good medium ground is to come up with a form-letter "thanks for your kind thoughts" response to send in reply, that keeps things fairly impersonal. Don't respond to anything harrassing--in those cases, simply report the person. And of course, don't accept friend requests from "fans" on your personal FB page.