If concerning a certain topic, then I would usually leave it behind. If I have nothing to say, then I wouldn't say anything as it wouldn't be much of a contribution or help. Usually, I would think it over after a certain period of time and later get back to it.
However, if in discussion with close friend, then I would usually respond with a strange emoticon or symbol. For instance, when nothing more was needed to be said, I continued the chat with |.| meaning stuck between two walls.