Reasoning definitely isn't because people are on snapchat. I've noticed a big decrease in engagement too on my large accounts. I fear IG are decreasing natural engagement for when they really give their ad platform a push.
I feel the same way. I've somehow predicted that Fb was going to more or less "destroy" Instagram. Seems like FB is letting IG drifting away from their initial "philosophy".
So many people are complaining about a decrease in engagement, pretty sure something internally is happening which we haven't come across yet.
There are two possibilities:
1. not appearing on all your followers feed (which would be really shitty and destroying the initial thought of why IG was actually developed)
2. not appearing on the explore page (which IG actually never guaranteed)
Even when it comes to the FB rate, some users have already complained that you don't show up as a notification on other user's profile, which might actually be a pretty good explanation for a decrease of the FB rate if something else hasn't been changed, but I haven't actually experienced not appearing on ones notifications myself.
Since I don't follow private users I hope that everyone's being notified, 'cause if you follow private users who have some requests to approve, I guess they're just added without you even being informed who followed you.
I can somehow see the whole explore page section being nothing but ads in the future, too.