Facebook is not about sharing
To most Facebook users (me included) Facebook is not about sharing - it is about staying close to their friends. Sharing is just a means to do that. When they share something, they open a window through which their friends can see them. Their friends might pass by the window just to peek in, or maybe they'll even stop by for a second or two, giving a thumbs up or dropping a comment: "Nice done!" or "Happy birthday!".
Photo credit: Alejandra Mavroski
The purpose of opening a window is to bring someone closer to his or her friends. That makes it less important what they share; they can pretty much share just about anything with anyone. If a friend doesn't find something they share interesting, the friend will simply ignore the window and move along to the next. There is no need to tag what is being shared to make it easier for recipients to filter their activity streams, nor is there a need to "neatly organize your friends into buckets" for targeted sharing.
That's the beauty of if - the simplicity.
Although the activity feed is messy, with important stuff mixed with trivia coming from any direction - current friends, acquaintances, colleagues, ex-colleagues, classmates, family members, childhood friends - it's still simple. You read, you like, you comment, you post. That's about it. You can even choose to only read, to lurk around. Your grandparents can do it, and so can anyone. That is one reason why they continue to use Facebook, but the main reason is that Facebook helps them come closer to you and other friends, and sharing is just one of the means to do that.
Note: A study by the Pew Internet & American Life Project found that active Facebook (who uses it several times per day) has on average “9% more close, core ties in their overall social network compared with other internet users. Facebook users also tend to friend other users with whom they’ve actually met in real life; the average Facebook user has never met only 7% of his/hers Facebook friends.