Here's my theory: most people don't actually understand what having access to your data means. I learned this some years ago when I wrote a little tool that surveyed the people connected to my uni's computer lab (using ssh and who).

The whole project was just for fun, and one of the things I did was logging the names and times of everyone who started a session. I then told about this to some friends, who were like "cool!". But when I showed them the actual data (rows and rows of "this person was on this machine from this hour to this hour"), with real names, they freaked out. Showing them the data they didn't learn anything new, since I had already told them exactly what I was doing. But it wasn't until they saw with their own eyes the real names and times that they actually understood the implications.

And this wasn't your run of the mill facebook user, it was computer science students. So I can understand perfectly why people don't care about online privacy: people just don't get it by being told about it. They have to see it.

So show them their usage logs, and they will understand