Non-American here. I believe US citizens often have a middle name. We Spanish-speaking people have 2 last names. Japanese reverse the "firstname" and "lastname". I had a lot of troubles when I tried to buy a ticket in a Chinese website a few years back. So no 99% of the users don't follow the "firstname" and "lastname" pattern. We might put up with it, but it doesn't suit 99% of the users as stated.

To clarify the comment you are replying to (not the author): 99% of their users. Clearly 99% of people globally aren't going to follow that custom, but many (most?) applications written in the US have a user-base where 99% of their users will follow the custom. Switching to something more globally applicable isn't necessarily a non-zero cost either - using a single name field has downsides like making sorting by last name more difficult (a ubiquitous thing in the US) and not being able to display first vs. last name where culturally expected.

But I am replying exactly to that. I don't believe 99% of the users of a given application for any non-trivial number of users conform to have exactly a "firstname" + "lastname".

In the US it's not rare to have a middlename. And there are many Latinos, Japanese and Chinese in the US as well, all of them which do not follow the convention.

A lot of US-based startups target the global market early on, although they behave as if they only had to cater to US users.

As a side note, the group of HN commenters seem to fall into two categories, the first think group of commenters seem to think HN is a mostly US only forum, whereas the second group seems to think HN is an international forum. I don't know what the truth is or whether there is one at all. But I know, that it sometimes irritates me, when commenters or posters assume, that they are talking only to US citizens.

I suspect it's usually accidental but I feel very little rancor towards them. You solve things the way you think about them (until yesterday it wouldn't have ever struck me that a currency symbol could be in the middle of the value). And that's how, I suppose, you end up with a bunch of people with US Visas that say FNU (first name unknown) in front because they only have one name.