Another interesting question (probably not interesting and solved millions of times before me) is what exactly is a trending note on nostr. Is it one with a lot of replies? Reactions? Reposts? Zaps? All of the above? Any of the above? A specific combination of the above? How much does it matter WHO did the action? nostr:note1wglva934zxlpvfsgq93vfqc6w6r33c89zkf3h6da9jn7tw3yw2wshuthqa
Discussion
There could be "discovery services for trending notes" and clients can choose from them and further apply their algos. These services can be part of relay software too.
I think nostr.band uses their Trust Rank to calculate a score that will be used to identify tending notes, profiles etc. Have a look here: https://trust.nostr.band/