I have yet to see one single ad anywhere on the Steemit platform. Isn't that awesome! I don't know if Steem has future plans to add advertisement, but I sure hope not.
So if there are no ads, where does all the hype come from? From media sources that mention Steem, SBD, and BTS for sure.
But a lot of traffic also comes from organic search results. I learned early on that you have to have a certain amount of steem power to upvote your own post, or get upvotes that lift the post past a certain threshold. Once a post passes the threshold, it goes from nofollow to follow. Follow can be indexed by search engines.
Not sure exactly the number, but this post by @cryptoctopus helps explain.
So yes, good content is extremely important. It attracts new members through search engines, which in turn brings new investors. As for seo, it's the comments that really help lift rankings more than the upvotes. :)