That's why I often ask for "Source?" — because sometimes people seem to make up numbers. However, whenever I do this, I receive a large number of downvotes. Maybe it's not common on HN to back up claims with sources.
IMO putting an important number in your post/comment, and not providing a source for that number, is also kind of low effort. If you verified the number before writing, you already had the source ready and you could just put it in the comment. If you wrote the number from memory, not checking if your memory is correct is low effort (but you can also warn the readers that the number is from memory, that's better). If you're intentionally misrepresenting what the number means in your comment (and giving the source would contradict the meaning of your comment), or just giving a number that "feels right" or a number that you know is wrong, then it's low effort and a lie.
I try to verify important numbers and facts in what I read, and seriously, there's so much fake or misrepresented info everywhere, on every political side, that it's depressing, and it makes me don't believe literally anything without a source, unless I verify it myself. Of course when someone provides a source, I often look into the source, and sometimes it turns out that the text misinterpreted/misrepresented the meaning of the source. On Wikipedia, I also check if what is written is actually in the source, because sometimes the editor writes his own opinion while only loosely basing the text on a source (or basing it on nothing).
Verification can take some time, and that's the effort passed from the author of unsourced claim to its many readers, unless they just trust it or ignore the claim.
When I write anything I try to include sources for important things. If I wouldn't include a source, and someone asked "Source?" I wouldn't think "what an annoying guy", I'd think "oh, I could have linked that in the first place". And I usually upvote "Source?" comments (unless it's a thing that anyone can check in 30 seconds). I usually double-check the facts in what I'm writing, and many times I almost wrote something from memory that wasn't true, but looking for a source saved me from that.
I would imagine it's more you're being skeptical of something that is unpopular to be skeptical about. It's like someone saying climate change is impacting our planet, and then asking "source?" in response.
No, that's not correct. I ask "Source?" when someone makes a claim that goes against popular belief, such as: "climate change is not impacting our planet." I do think "Source?" is generally considered a low-effort response, so it's the wording I guess, not the context.
He was asking about the trend of him commenting with "Source?" leading to downvotes on other posts. I was giving an extreme example to represent why that may be happening elsewhere. I agree that in this case with the 3% rate it's likely not applicable.