What I've always assumed, but now this thread has me doubting myself, but what I've assumed is that these systems, even though they appear to suck at specifically targeting me, must somehow be pretty good on the average, still netting big profits overall even though they don't seem to live up to the promise of getting me to buy stuff. But if everyone has this impression, maybe it doesn't work? I mean, I assume companies like Amazon and Facebook would be pretty good at least at optimizing the total rewards from using recommendation systems, but maybe you can't tell from your own anecdotal use. I have no idea. I'd love to see an analysis that includes aggregate numbers.

I think you've got the nail on the head. It's sort of the tragedy of the commons, which in terms of recommenders is a really easy thing to accidentally optimize for. For example, pop music is popular. In the early days, recommenders would just recommend pop music because on average that was a decent recommendation. We've come a long ways since then. Well, everyone but Facebook has.

> For all Facebook knows about me they've always been exceptionally bad at advertising to me

Because it's not Facebook really, it's the advertisers who choose targeting criteria. You as an advertiser have a myriad of options. For example if you've built a competitor to X, you can target users who've visited X recently, aged N-M, residing in countries A, B and C, and so on. There are options with broader interests too. Poorly targeted ad means poorly selected criteria by the advertiser (or sometimes just advertiser experimenting) and consequently money wasted. Facebook doesn't care though.

Then there is retargeting/remarketing (target bounced traffic) already mentioned here is probably the stupidest looking invention that actually works.

But they are also really really bad at curating content I like to see. Like even worse than the advertising actually. My feed is just garbage and has been ever since they switched over from being chronological. But I sometimes get on a wild hair about some particular person, and I'll look at their page directly and see actually interesting content on their page that was never shown to me. Facebook's only real guess is then to just say: oh! You must like this person, let me show them, all the time.". The reality is, I'm interested in stuff like when a biased person I don't like shares a more neutral or inclusive opinion, or someone does something interesting. Facebook is just unuseably bad at selecting for that. Their algorithm just pushes really shallow crap and buries anything with substance or depth. I keep it around just to stay in touch with hard to contact people.

Likewise Pandora does a decent job of picking songs for me based on what I previously liked. It's not perfect but far better than random.

it's easier to recomment things you probably like than things they want you to like/may like