Not only that. I use the mobile interface rather than the app for two reasons: Sure, the app is badly done and buggy, but, most importantly, it gives Facebook permissions that they've shown time and again they shouldn't have. They've fucked up on more than one occasion, and I know casual users who balk at installing applications that request too many permissions.
That's definitely a contributing factor, in my opinion.
I haven't used iOS in a while, but a couple of years ago the web app was faster (!) and had more features (like liking on comments, not just on status updates) than the native app (presumably because the browser used a JIT for JavaScript and apps an interpreter). I would use the web app for everything except uploading pictures.
While some of the audience will be because of that, I doubt the majority are. Last time I checked, Facebook's mobile site actually degrades fantastically well, all the way down to old Nokia feature phones. You can bet that in many countries that is a big deal.
Bingo. Outside of Tech Companies/Silicon Valley/Major US Metro Areas/the US as a whole, iPhones don't have that huge a market share. More people have Android phones, but often running old hardware, old software, and even more have feature phones. If you're reading Hacker News, forget that you're not a typical user at your peril.
That's definitely a contributing factor, in my opinion.