Every Single OneI do try to respond to all comments made on my posts. I figure if you have taken the time to read and comment then you at least have earned the respect of having that comment acknowledged.
I don't always reply to private messages or whiteboards, especially if you are not in my circle and there really is nothing I can reply with.
"Hi, like your pic, sexual comments, and add me's", usually get ignored. If they are conversational I always reply after checking your profile if you are not in my circle.
But I do read every single one of them.