I have times with messaging that I don't have time to respond....but a hi/hello is quick....but if I'm in a hurry many times I don't see a PM at the time I'm trying to get off the computer & end up not seeing it until I come back on whenever that is. Many times if a PM requires a long response, I will give a quick reply to let them know I read it & explain that I don't have time at the moment to give them the reply I need to give & will get back with them when I have more time.

That way they don't feel like I'm ignoring them hopefully.