> The cohort of young people that used AIM/MSN/Y! (depending on your region) aged up into Facebook Chat [1].
Speak for yourself, most of my social group is split between iMessage and WhatsApp.
Even so, what Facebook has that MySpace didn't is enough money to buy up its competitors to stay relevant (e.g. WhatsApp). So this might be what will happen, even if Facebook itself fades in relevance.
(Bear in mind that even if Facebook Messenger grew in popularity as AIM declined, so too did the number of people online increase, and the number of people with smartphones -- saying that one increased while the other decreased is not sufficient.)