it's natural to expect privacy.
It's natural to some of us. It's apparently not natural to some other people, which is why they broadcast their stuff to the Internet.
If you're fucking your wife in your bedroom, you expect privacy. If you're fucking her in the town square, while occasionally making eye contract with strangers and saying, "hey, check out what we're doing," then that suggests that you don't expect privacy.
I think the better rhetorical question is: why are some people so amazingly stupid, that they are incapable of telling the difference between these two scenarios? What is causing this stupidity? Is there anything we can do about it, and if there is, should we do it?
The real problem for the facebook posters is that on the internet, human culture doesn't apply, and they have yet to come to terms with that.
No, the problem is that some users don't know the difference between fucking in the town square (uploading to facebook) vs their bedrooms (sending encrypted email).