Inevstors and traders are mostly active on twitter, if you don't have much fund for Ads placement on popular omega sites. Tweet, retweet and tagging of friends might do wonder and the cost is minimal as compared to placing Ads.
Paid advertisement is always better than the free advertisement. I don't think an exchange could be low on marketing funds. I think they should opt for paid advertisements which will give them far better results in quick time.
Those exchange who rely on free advertisement cannot get any good result since if we talk about advertising they will only get limited traffics since they will not reach up for those real investors or traders itself. But if the site has prepared some good amount for proper advertising for sure they will get a good result especially if they offer also a good promotion,competition and hiring some good influencer to market the exchange.