And don't we think there will be shortage of users here as time passes?
Bitcointalk is a forum to discuss about Bitcoin and other whatnots, what would be the benefit of the forum spending money to run advertisement. Notwithstanding the number of users here, there is no financial gain for the forum. Bitcoin is quite popular around the world, and for the most people who find out about Bitcoin, it does not take long before they discover Bitcointalk and register on the forum. There are old users who have left the forum, but new users register everyday, though what we miss is the knowledge of some of the old users, and not the number they added to the overall traffic.
Having said that, because of Bitcoin, businesses like exchanges, mixers, gambling websites (that accept crypto), etc, have all sprung up, and there is no better place for them to advertise their business than in the biggest Bitcoin community, thus the users here can earn money through posting in the forum, and that is an added incentive and a reason why the traffic remains pretty high. I think if there is ever an end to paid signature campaigns, that's when the traffic in the forum would really go down, and we'll experience shortage of users.