Well unless I have a specific question I'm joining to ask, I always lurk a bit on a new forum to get a feel for the culture of the board before I actively start posting. If it's not my cup of tea, I don't post OR lurk, I just don't hang around anymore.
Even boards I'm registered and post on, I read way more than I post. So often someone else has said what I would say, or I don't have anything to add, or don't know much about the topic, or just don't have time, that there's not much point.