[drbawb]

This fact alone may also add a bit of credibility.

If they were originally wall posts, your "threads" of conversation would be comments, not individual posts on each other's walls.

[bwaldrep]

Not true. This is the source of all the confusion. Originally you couldn't like or comment on wall posts. Conversations were individual posts on each other's walls. You could click the "wall-to-wall" link to view the whole conversation.

This system gave conversations a different feel, and is now causing people to confuse them with private messages.

Edit: see http://news.ycombinator.com/item?id=4566956