Well, posting on your wall is not equivalent to having a private conversation with a single friend, it's equivalent to chatting in a large group where anyone can join in. I rarely have "conversations" on my wall, but for example: a colleague of mine is now in Vietnam on business. She's never been there before. She has a few snapshots and random observations of interesting things she's seen or done, but instead of sending them all collected as a letter to one specific person or more, she just posts them on her wall in case anyone feels like commenting. And we do, and I think it's fun to see my other co-workers comments. It's just an informal group conversation. Actually it's pretty much like posting here.

But people only tend to post on their walls if they actually get feedback. Posting "it's snowing out!" is only fun if you have friends who are yearning for ski season themselves, and will join in in praising the snow, or conversely you know you have friends who loathe the snow and will tell you so. If you always post to express your opinion without expecting or assuming you'll ever get a response that seems self-centered to me. But that isn't really the norm.