Funny you should bring that up, because FB is a perfect example of a visual relic from the late '00s which didn't age well. Much of the post-dot-com-crash world of design in the web and early mobile space was defined by a rejection of the excessive - wild crazy idiotic irreverent and often incoherent - typography and design of the '90s web. So, super basic fonts, white backgrounds, and what they thought was "minimalism". However, because they relied on the tooling rather than creating their own cues, (and also because their UI turned into a towering nightmare of crap) Facebook looks extremely dated to most people's eyes now. It looks like a 20 year old website...and not a particularly good one.

My outside opinion is that Meta does not give a shit at all about how Facebook looks at this point. I'm not here to make the case that revolutionizing their design would improve their onboarding rate now or something. I'm making the case that if you are some nobody with no network effects and no billion dollars, you have a better chance at getting people to read your content if you take the time to (a) get it out of system-ui fonts and (b) make a bunch of other aesthetic decisions that trigger certain pleasing feelings in your audience.

Most current system screen fonts in major OSs - as a general group, as a set of letter forms - are not and never were printed fonts. They evolved from bitmap fonts that were created with 90s constraints in mind. They retain some of that aesthetic, and they are not usually the best candidate for the job.

Leaving it up to your user's system font is also an arguably poor choice.

> Could you recommend any good books

The Elements Of Typographic Style by Robert Bringhurst

"Five minutes to learn, a lifetime to master" kind of thing.

>> If I saw a flyer pinned to a corkboard advertising a fish and chips special in Comic Sans in a seaside town, my mouth would be watering

Heh. You absolutely nailed the concept. This is precisely right. Comic Sans is a totally crap font, but it's great if that's the feeling you want to get across! That's exactly how thoughtful design works. Level one is to know your references. Level two is: Use the kitsch when you want it. Use the profane. Use the found art. Then use the "classy" or "expensive" when you want to show a "classy" client. Level 3 and up is knowing how to play with those high and low cultural notes to make a kitschy client still look kitschy but subconsciously implant that the fish is of a slightly better quality; or make your "classy" real estate agent still look "classy" but show that they aren't a stuffy asshole. And do this without either client outright complaining.

Far as books, I'd qualify by saying that I learned the design craft on the job from an early age and was tutored by great art directors who let me experiment and explained these things to me; why what I was communicating with a certain background or certain font was going to negatively affect a consumer in a way I hadn't considered. And I had a couple great professors in college (but I dropped out). Most of the basic theory of the higher level of how to use this stuff to influence people is contained in Marshall McLuhan's "Understanding Media", which is the quintessential work on how one - in a design role - can change perception of the content of a piece by making informed psycho-sociological choices about the way it's presented visually. Lots of master works before and since on the concept of propaganda, but for designers McLuhan helpfully narrows it down to show that presentation is often more important than content, and for designers this means your role is essentially to master content the way an audio engineer masters a band's album. Which is often as important to the final output as the original recording quality itself. This is especially hard to do if you also wrote the content because it requires a certain objectivity about the work. Anyway, McLuhan's theory is timeless and kind of essential to get into the guts of everything: How brands work, how small design choices work, etc.

Being able to explain the difference between what looks good and being able to explain why you chose it (along with all the cultural references, positive and negative, which the choice is intended to trigger in the audience) makes the difference between a $40/hr design job and a $150/hr art direction job, so mastering the technique of selling the technique has a certain benefit as well. But you must come armed to your client with examples. The best way to learn is to look at everything you see around you and question all their design choices, why did they use those colors, those fonts, etc. and also see what trends are developing. Go back to the 1950s, 60s, 70s, 80s, 90s and see what they were using then. Now who is your target audience, and what feeling do you want to give them?

[edit] Just want to add: I was a purely technical child who learned to code and run a BBS way before I knew anything real about design. I came up doing what I thought was cool ANSI art and then "designed" some stuff in Photoshop 3.0 for people when I was 12. I didn't understand anything. When you're a technical person you tend to see design as a problem to solve with programs and with code. You don't realize that design is a programming language that runs on human brains. This was a revelation for me when I was about 15, and it's informed all my choices since. So yes - fonts matter.