Group chats are basically the Circles that Google+ saw the need for but could never get fully set up. A lot of people don’t want to share personal updates and photos to a broad swath of friends and acquaintances.
Meanwhile Instagram and Facebook keep evolving. Facebook is turning into a weird Reddit for older people. Instagram is turning into a hipper LinkedIn, where artists, musicians, and local businesses share career and business updates and advertise their wares.
But sometimes I do, because saying something to one person feels like I'm demanding a response from them, but saying it to a broad circle of friends allows those interested to reply, and others to leave it. Back when I used Facebook, I was more likely to gripe (or brag) on the Facebook wall than in a personal text conversation with a friend.
(Friends in person are the best option, of course.)