As a none American I point out that WhatsApp groups have functionally replaced old Facebook for a long time.
I completely agree, my only issue with WhatsApp is that when I get a message, I need to read it immediately and its 50/50 whether its important or just a random link to something. For me at least, the reason Instagram messages work is I can ignore the notification because I know its definitely not important and just something funny to look at later.
My friends and I have silly group threads but they fizzle out fairly quickly because everyone is worried I think that they could be bothering people during the workday with a rather high-priority notification.
Instagram is better, like you say, because we can absorb the links and gifs passively.