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.
kucoin buying many different crypto related facebook while okex is hiring many people that have groups or have people with community to announce thier giveaway i know many people hired to complete those task.