As in people you know or have maybe just met once or twice, or even just stalk on Facebook, but are definitely not intimately involved with. Have you? Would you? Is there a difference between doing it over a mental image and a photograph?

I haven't, and I don't think I ever would. Just seems a bit... umm... rude. I don't really have much else to say on the matter, or anything in the way of explanation. I'm more curious to know how common a thing it is. And whether anyone will actually admit to it.