When this issue came up last time, it seemed as if the only reason fbcdn images were ever hitting Google Image Search was that people had deliberately hotlinked to them from their own web pages. It is actually just as easy --- you could argue that for a non-tech-savvy user it's even easier --- to save and repost the photo itself.

I agree with your analysis though: once you post it, it's out there, and there's nothing you or Facebook can do about it.