Maybe most people are imagining seeing banners on Steemit. But let's remember Steem is a content publishing platform and, as such, nay be that advertising could be implemented as "promoted posts based on your previous interests and activities on the Steem blockchain".
Airdrops are often published on Steem. Those companies could create a new user account on Steemit and pay them in order for the post to be seen by those users which upvoted posts containing airdrop word on the title and which post or comment in ETH or Waves tagged posts.
I don't use Steemit too much, but I won't stop using any Steem client which started including that kind of promoted content.
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