Lurkers can be practically inconsequential from the activities of registered users when it comes to monetization through advertising networks since they help inflate a forum, blog, or other website's page view count just as much as a registered member is likely to. The only thing that is lacking with them is the fact that they do not often contribute to discussions, create new threads, or iron out any content of their own for the site. They are just there to - like their name suggests - lurk. If you are creating a forum in the hopes of eventually generating some money, then they don't really matter. In fact, they are beneficial compared to the average visitor who will browse a site for a few seconds to a few minutes before leaving.