I've always been aware of iCloud Photo Sharing, as it is a great way to share photos and videos with a small team: your family, the friends you went on a trip with, etc.
A couple of weeks ago, I gave my 7 years old daughter an old iPhone, stripped down to the basics using parental controls, without a SIM card and configured to only access our home wifi. It is more or less a glorified iPod that can take photos and videos and can send iMessages to her parents (when in wifi).
Yesterday, her mother showed her how to share photos and videos with us using our "Family" shared album on iCloud.
Daddy, I gave you a Like.
And so I realised that in addition to sharing photos and videos, you can also like and comment on them. And you also get a nice activity stream and notifications.
If you use it like this, iCloud Photo Sharing (see iCloud: Create a shared album with iCloud Photo Sharing) is something like a simple, ad-hoc, private Instagram accessible from all your iOS and macOS devices.
There's also a "Public Website" option, that will make your photos available via a public URL, like this one, but you don't get any interactivity there (likes and comments).