It's just unusual if you think about it. It helps to have been alive in a time before the web was significant. Think of all the companies you give personal information to. Think of how much information you actually give them. Think about what they are permitted to do with that information.
Then think about sending a list of all your friends, family, colleagues, your personal thoughts and commnications on personal matters, to a random, characteristically anti-social CS major you've never met. It's just plain weird. And Zuckerberg was absolutely right in what he said: we're dumb to do it. The problem is this didn't phase him one bit. He went right ahead as if it was all going to be OK. And we just kept sending him more and more info.
Granted, Facebook calls itself a company, and the situation has grown quite large and complex, but I still think of Facebook as one CS student's website. It is what it is.
You send your personal info, in some cases more personal info than you would give your personal banker, your doctor, or your lawyer, through the web to total strangers, many of them are anti-social kids like Zuckerberg, to be posted on his website.
Maybe it's because I've grown up with fairly ubiquitous internet, but I've never expected anything I put on the internet to be private. Furthermore, I've never expected anything done in public to be private. I have a hard time understanding why people expect otherwise on both.
The idea that you should or shouldn't "trust" a company...