Steem is posting rate limited blockchain. It means that instead of paying fees to post, every user can make posts for free based on how much Steem Power they have. The more SP user has, the more frequently he can make posts.

https://bytemaster.github.io/article/2016/02/10/How-to-build-a-decentralized-application-without-fees/

If the blockchain starts to grow too fast, we can just change the limits bigger. Normal users won't be affected much because they don't need to really post very often. And if they need to, they can buy or earn more SP.