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.
This is spot on. Facebook proper has supplanted private email chains for a lot of older people. This is ironic because they are moving in the opposite direction as everyone else. Everyone else is moving into private communities, older people are leaving the safety of email chains and, often unknowingly, posting publicly. Facebook (probably intentionally) upholds the illusion that they are posting for their friends. I've seen Facebook actually provide a compelling service to my older dad who keeps in touch with a lot of his old friends on there. It's a much more active community of seniors than you'd guess.
Of course, they are subject to all the ills of Facebook at the same time. Overall I'd rate it as a net loss for society because of that.