* saying "fuck it" to Facebook and using Instagram as their primary means of communication to "everyone" * using SnapChat or WhatsApp for other communication * people are actually starting to text.
When I look at my FB feed, I see the posts of 10-15 people max. I do check people's pages just to make sure FB is not fucking me over (and sometimes it is) but most of the time, people just drop off facebook. They're not "leaving" but they're not using it.
Facebook is more for people you know. (Filled with babies and relatives.. etc) where Instagram has been great for finding like minded folks and following along with their lives in short snippets.
As for how to communicate, Instagram has user tagging. Even in others comments. Basically the same as sms messaging.
( My wife has more followers than me and I need to catch up: http://instagram.com/wandering_tim )
So for instance, I have a friend who posted a picture of her new apartment. That spurred a discussion about it. Pretty simple :)
It's actually more convenient than facebook, at least I found it so. And I don't have to read anyone's stupid post about "I wish it'd stop" or "Never again" or "You're a bitch blah blah blah blah blah. You know who you are" -.-