Steemit monetize its platform with advertisements. And it uses the following ways to earn money.
Steemit makes money when its users invest into Steem Power so their posts can be seen by a larger audience, thus getting more votes which translates into more money. Steem Power is locked for two years, so theoretically Steemit could use that money as an investment in the platform by which they can make a larger return than what they’re giving back with rewards.
Additionally, they use the blockchain business model, their tradable Steem token can appreciate in value, which will bring in a source of cash for the company.
The developers have also held a portion of the tokens, which they can cash out or reinvest back in the business.
Steemit also allows its users to promote their posts to reach a larger audience. Steemit has its own built-in way to promote each post. This is not unlike other decentralized platforms such as Minds, which also allows users to boost posts this way as well. Think of it like a Facebook Ad, your essentially paying for people to see your content, hoping that will convert to sales for your business, or in this case, upvotes and earning potential for your post.
The best way to think about posts in Steemit can be summarized by the following statement from this article,
“Steemit shows ads and you can imagine each post as a single advertisement. In order to attract attention or “sell the contents”, one should invest his money. If people like your post you will get your money back and even earn something. Other content creators will lose.”