A steady flow of people
Emerging from a cave.
Up to two score a day,
And usually far fewer.
I originally thought this referred to the number of new users on Puzzling.SE but it turns we get more. After the author's hint in the comment, it's clear that it's about votes; you can get a maximum of 40 votes per day, but most people don't use them all.
Some go down the cliff,
A brief two-metre drop.
You get -2 reputation if your post is downvoted.
Some prefer to climb up,
Five or ten metres each.
You get +5 or +10 reputation for upvoted questions resp. answers.
One might help another
To ascend even further,
Beyond a dozen metres.
Accepting an answer (which is stored as a vote in the Stack Exchange database) nets the answerer +15 reputation (which is more than 12).
Sometimes with a pulley,
One goes up, another down,
But balance is retained
In this quite rare event.
This could be about unaccepting one answer and accepting another (which gives -15/+15), or about bounties (which are also stored as votes). Note that sometimes, bounties do not retain balance; if the bounty is automatically awarded, the award is only half the invested amount.
You've seen all of this,
And taken part in it too.
Everybody who visits this question/answer sees the authors, and that both question and answer have been voted on. And most people (but not all of them, e.g. unregistered users) have voted as well (not necessarily on this question or answer, but on other posts, or elsewhere in the network).
So who are these climbers,
And what's the cliff they're on?
The climbers represent votes and ... eh ... reputation?
