If they plan to become known quickly through the forum, they have to spend money for ads including the forum ad slots.
i agree promotion is a nice way to have more user's and there's no shortcut to be popular exchange but having many kind of advertisement can help them to be known by user's.
I remember okex and kucoin do a promotion in social media such as facebook and sometimes they run a give away to attract new user's to registered and participate in thier exchange.