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I think you do need some sort of ranking to keep out the dross or mega boosted posts from polluting the front page.
At some point I'll introduce a high fixed cost to boosting rather than letting it be arbitrarily set.
How would the downvote work? Would sats be taken off someone or would the post just lose rank?
It depends on whether it's in the trusted or trustless model. In the trusted, the post would just lose rank. In the trustless, the downvotes would do both: if they post does poorly (other people also downvote it), then the downvoters get the sats else the poster gets them.
Could account age or regularity of visits be a useful factor in the ranking system? Although I'm sure there are probably ways they could be abused as well.
There are a number of metrics we can use like these to quantify trust. They are kind of arbitrary though ... That's why we'll probably do WoT.
I assume it's difficult to create lots of spam accounts because a separate node would be required for each one? Or maybe I don't understand LNURL well enough!
Nah, it's pretty inexpensive to spin up new nodes/wallets. LNURL-auth just requires a key pair ... we could do something to make sure users have funded channels or something but we can't expect all users to be running ln nodes in the future.