1) When did you start using it? I remember starting quite early, so maybe the well-maintained garden comes from the fact I did it gradually over the years, so I didn't notice the work?
2) It may be simply that Facebook caters well only to some lifestyles, while it conflicts with another. And/or personality types; I think I've noticed that the more extrovert people tend to use Facebook a little less - mostly to share funny stuff and coordinate the next real-life party. All the interesting discussions I have on Facebook are usually held with people who, like myself, aren't very party-going.
I never really "used it" for any length of time, just a few weeks or a month or two. This was probably 5 years ago. I still have the account, but I log in maybe once or twice a year. I basically accept everyone I've met IRL, because it'd be rude not to (right? I don't know the etiquette). I know there are muting controls and stuff, but that's already more effort than I care to spend given I see no benefit.
I'm quite introverted. I'd say I have about 10 people I'd call friends, and even fewer that I talk to on a monthly basis. I have my own sources of finding entertainment, and I don't like parties; I go to three or four a year and host one or two myself. That's plenty for me.
It just doesn't fulfill any purpose in my life and actively makes me angry.
(To be clear, I'm just describing my own situation, not expressing any sense of superiority.)
I don't like framing the issue in terms of "missing out". It leads to the "paradox of choice"-type worrying. There are always more interesting and fulfilling things to do than hours in a day. You seem to have developed a good social life framework for yourself that doesn't include Facebook, so I wouldn't say you're missing out.
As for stream, if mine looked like that I'd be unhappy too. You could probably clean up yours quickly though - just scroll through it and/or visit profile pages of some of your friends, and start "liking"/commenting on posts that you find interesting. Facebook filtering algos will quickly catch on your interest, and you'll be seeing much less crap.
RE etiquette, as far as I can tell you're correct; generally you accept people you met IRL, with rare exceptions of both kinds - sometimes you don't really like your new acquaintance, and other times you may decide to add someone you met on-line if you know them well enough already. At least that's how I myself and people I know use Facebook - maybe the culture is slightly different in other parts of the world. But the rule of thumb seems to be: if you think the person is OK and you might want to talk some more in the future, it's OK to send/accept a Facebook friendship invite.
> It just doesn't fulfill any purpose in my life and actively makes me angry.
That's totally fine and, IMO, a perfectly good reason to not use Facebook.
It may sound crude, but it is pretty effective. They make sure that you mostly see things you're interested with, sprinkled with some other stuff - because a) you might find it important too, but they don't know, and b) everyone changes their interests from time to time.
[0] - Yes, there're options to signal "please don't show me this kind of stuff" or "don't show me stuff from this person", but I use them so rarely I don't really know if they work in any other way than just blacklisting the thing/person you pointed out.
[1] - I've noticed this directly after liking a post under a fanpage I basically never interacted with. I suddenly saw their posts everyday. Then every other day. Then every few days. Another like, and they're back to showing up daily for a while.
Facebook is a different thing for different people; I think it's as good to you as you're willing to maintain it. Also, if you have crappy friends in real life, you'll have crappy friends on Facebook.