In response to a post there are numerous ways to provide feedback. You can reply to the persons post in the thread. You can give that person rep. Or you can send that person a visitor message or a private message.
My question to you all, is what determines what method of feedback to give to someone's post?
As for myself, I'm most likely to reply right to the thread if I my response to someone's post is also likely to be of some interest to other forum members and if it adds something to the thread that wasn't already stated by someone else.
I'm most likely to rep someone if I thought their post was particularly well written or amusing in some way. Sometimes I'll rep people rather than replying to the thread if I agree with what they say but I don't wish to clutter the thread with a "me too" post. In some cases I may both reply to the thread and send someone a rep.
I rarely use visitor messages or personal messages as a means of providing feedback to someone's post. I will use visitor messages to respond to someone's rep comment or ask them a general question like "where did you find that avatar?" I almost never use personal messages. I only use them in the case when I don't want the response to be seen by others who view that person's profile.