I use it to track my reading and to look for more info on books / authors I'm interested in. I use Calibre, too, but for me, Goodreads is more "what I've read and a few things I definitely want to read" whereas my Calibre collection is more like a book repository to feed my devices.
I belong to a few Goodreads groups. One of those groups is an unofficial offshoot of Mobileread that was started when a few people wanted to go more in-depth with the annual challenges. I also started a group called Reading Gamers that's, well, for gamers who are also readers.
Groups are really like mini-forums rather than your typical social media interactions.
As for the more "social media" type features, my use of them is incidental, really. An occasional comment on a friend's review or a like when they've shelved a book I enjoyed. I hardly ever look at the activity feed, I usually go straight to the page that shows group discussions to see what I've missed.