I know that this sounds curt, but sometimes you are not the intended audience of a Facebook posting. People many times post something so that certain people and/or groups of people know what is going on.
A good example of this is that I posted a concert date on my Facebook page of a high school classmate, who is now a Broadway star, is putting on a show in our area. While I don't care who sees (and responds to) the posting, it was more intended for my high school classmates and those I am Facebook friends with who are into the arts. Unfortunately, the way Facebook is set up, it can be very difficult to direct a message to a specific audience.
I was guilty as well of responding to a post made by a friend last week regarding a charity that he is involved in. I told him I was willing to make a small contribution to the charity, whom should I send the check to? I never heard back from him. I think it may of been due to the fact that I wasn't the audience of his post.
I am not sure if NTs automatically pick up on that they are not the intended audience of a wall post, but I have noticed with myself and others on the autism spectrum, that when a conversation sounds interesting, we involve ourselves in it.