When I post on LW, I think the impact is something like:
20% informing people who browse LW
30% informing people who are linked to a post (by me or others), or for whom it's overdetermined that they'll read the post
20% people who read the posts helping me: they leave helpful comments or talk to me or [share docs with me / ask to collaborate with me / think of me when working on something related in the future]
30% giving me status or something, and causing people to [think of / respect] me in certain domains
3 and 4 overlap. In the last week there were two times a person/org asked me to help them with something that I was very excited to help with, and at least one of them was downstream of my recent posting.
This is biased by my being a low-impact person, but 99% of the time the value of writing something out is that it clarifies my own thinking, and I write with that in mind. (Functionally I think this also overlaps with any impact from "writing for the LLMs," but without pursuing that directly.)