It's not the culture. I care about what people think about me, but not as much as asking my friends to describe me it three words.
I think I understand it now. People use this, because they wanna hear people saying good things about them. I think most people would send link to their friends, because they don't feel very confident about themselves.
It's the basic principle of Facebook (or of whatever Facebook turned into) - people post things so they would get positive feedback and they would feel better about themselves.
Well speaking for myself, I am pretty clueless on stuff like whether people think the things I say are generally interesting or generally boring. I wouldn't be ashamed to use Mark's site to try and learn stuff about myself. Of course, I'm sure the factors you mention provide subconscious motivation as well.