Facebook had incredible granularity with regard to who could view one's posts — then they nerfed it by letting the people who are on your "lists" view who else is on them. (I'm private enough to not want people to know that they're on my Close Friends list with 30 other people.)
There was a way to get around this nerf by including a "smart list" in your set, but Facebook recently removed "smart lists" altogether.
My Facebook post privacy settings are a mess now, and I also gave up and stopped posting anything personal.
(I wanted to add that I used LiveJournal a lot in the early 2000's, and their friend list feature was amazing.)
I think it could really have replaced all the fragmented around Google comment forms, but it sure wasn't wise to force everyone on YouTube on it in such a shitty way. I think that initial public outcry was a significant stop to its velocity, yet it could have become moderately successful and very valuable long-term to slowly integrate all of Google's services. Just so many parts of the execution were questionable, including the mobile apps :\
Was the lack of a semi-immediate huge return the reason it was killed?
(I also suspect some of it comes from people looking at Apple's historic "make simple things simple" approach to UX and taking the wrong lessons from it, which arguably includes latter-day Apple, but that's a whole different post...)
Tangentially, it's still interesting -- and faintly depressing -- to me how much I think LiveJournal got right about social media that its bigger successors, well, don't, specifically related to privacy. LJ users had comparatively immense control over who could read and reply to posts.
This isn't to say that Twitter and Facebook don't get a lot of things right, too, in terms of ease of posting, lack of friction, and (for both better and worse) monetization, but I wish "granularity of acces control" had been a lesson we'd carried forward.
In any case, I'll be curious to see where Brad ends up next.