Except they did in 2011 in response to Google+ by adding Friend Lists, which you can use to filter your News Feed down to stories and news items from just friends in certain lists curated by you. This is still in existence today in 2018.
I disagree with the GGP (I think) about Facebook wanting you to wade throw a river of crap so you stumble on ads. Their ads have been spot on to my interests so far.
I think an onioned version of Google plus circles could help Facebok influence the AI without preventing discovery.
But TBH, had G+ really been a contender at this point? This sounds like the creak of an abandoned house by now.
The killer feature with circles is being able to post different things to different audiences IMHO, which Facebook can't do without different accounts.
And I want it simple and easy to understand and use, even for non-techies, such that it "just works". And a pony.
I never got the whole circles thing as a unique selling point... like that functionality existed in Facebook beforehand and the reason people didn't use it much is simply because for a personal social media it's not that useful. I've used it maybe a handful of times in 10 years.
I mean I guess maybe because circles are a nicer design concept?
Though let’s be honest here - anything you post on Facebook (or any other social media) you should assume is public, regardless of privacy controls.