> You can post updates visible only to a group of people
You can in theory, however in practice I've found it to be very cumbersome. I spent some time in 2011 trying to sort my Facebook 'friends' into groups - people I know from work, from tech meetups, and so on. It was too much work and the groups were too coarse. Facebook's graph search was promising, and I'd love to occasionally share a link (say) my policy-wonk friends will like, but there was no way to pair up graph search and status updates, e.g. share a link with "my friends who like technology policy". And of course, graph search is gone now.
I think Facebook's thinking is that you should just post an update and they'll figure out who'll be most interested in it. But the problem with that is there's no way to know who exactly is reading your status updates.