There are ads. Consider ever single post an advert (even though its not technically an advert). Content providers are given money by the crowd/whales for content that the crowd/whales deems appropriate. In the future advertisers might become whales in this system rewarding those that advertise content relevant to them, or they could pay whales to upvote their content. However the major difference will be that the crowd will be able to see if content is being promoted as those who upvote and their SP are publicly available. Thus resulting in a sort of free market for advertisers, content providers and users. How this free market will play out and whether this will work in practice we have yet to see.

Also this is also dependent on how large the user base is on steemit. The larger the user base the more likely that marketers, advertisers or even propagandists will use money to promote their content.

Essentially steemit is like a facebook without the zuckerberg. It has the opportunity to connect advertisers with users directly, without a third party such as Facebook.

One more thing is speculators, investors and market makers that provide liquidity for people to actually cash out/in. And also the inflation mechanism which incentives people to convert their STEEM into SP and SD locks up a lot of the supply, thus driving the price up and consequently getting more people to buy STEEM for speculation/investing, which in turn allows people to cash out from SD/STEEM.

In conclusion advertisers, marketers, speculators, investors, market makers and casual people buying into the system are the ones that pay content creators.