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One way of doing this could involve modifying the UI to recognize tags that a user tends to post or comment on, have those tags listed at the top, and randomly activate/deactivate them to generate diverse searches.

Of course it'll be possible to disable this.

Another possibility is to 'fold' time.

Now, 1 hour, 6 hours, 24 hours, 48 hours

Each loop amplifies the prominence of the post's curation for visibility purposes.

what we could do is base it on how many comments on avg the user gets based on his content. things like how many upvotes he/she gets. Or for some steem you could pay to be there for 5 mins or something like that

Those are also possibilities. Keep in mind that basing on comments may favor posts that foster controversy and arguing. :c)

Also, the steem for 'pinned' time sounds interesting. How does this compare to 'Promoted' posts?

from what i can work out "promoted" is for the full 7 days. pinned would be for a maximum of 6 hours.

I can see there being potential in exploring different possibilities here.

A little like google's ads at the top of searches. Maybe an auction for tag prevalence option.

yer it would also allow steemit to offer more money for it's posts. more money for everyone, lol