I think the definition of "imaginary friend" might be messing up statistics here? As a kid I definitely made up friends in my mind, but only as a way to make myself seem better in front of other people.
Other than that, is talking to an imaginary person any worse than talking to yourself in the shower, or re-doing past conversations with others in your head?
I will also offer similar anecdata. I had an "imaginary friend", but my memory is that he was a tool for spinning stories ("oh, yeah, I've got this friend who lives in New York..."). Pure roleplay, I don't recall that I ever believed him to be real.
Other than that, is talking to an imaginary person any worse than talking to yourself in the shower, or re-doing past conversations with others in your head?