Author here!
You're absolutely correct—and raise a good point. I think the important thing to consider is to take the same groups of people into account (e.g. if you look at exhibitionist/extroverted people in the younger, look at them in the older). It becomes tricky, because some behavioural tendencies can become exaggerated with age. Nevertheless, I do have introverted friends, but they still do participate in the charade, just in a more passive role. They still instagram, they still snapchat, they just don't generate much content.
This is still a marked difference from older generations who, as statistics show, avoid that entirely.
it is worth noting that this is looking at trends only—obviously not all young people are on [instagram, snapchat, peach, etc], and obviously all of their behaviour is the same.
One thing to note is the networking effect of these apps. I have a wide breadth of friends who share varying amounts about themselves and others—but the less their network is involved, the less they are involved. In your case, if a circle of friends is inactive on social media, there is probably something to be said about the effect of a weak network. Though, that's not to say that a strong network will compulse people to post selfies, either!
Thanks for the comment!