Abercrombie [...] became inseparably linked with Americans who came of age around the turn of the century. The price of being so closely associated with one generation was that the next wanted nothing to do with it.
Facebook, which took off around the same time [and] which began as a way for oversexed Harvard undergraduates to rate each other’s looks, is now seen by youngsters “as a place for people in their 40s and 50s”
Those aren't the same! In the Abercrombie case, the case being made here is that the same people are wearing it, but that cohort has naturally aged and younger people don't want to follow them. In Facebook's case, older people have joined the network over time -- the stereotypical aunts, uncles and grandparents on Facebook are late adopters, not the original users who have naturally aged.
Edit to add: to put it another way, if you think of a 37-year-old techie like Zuckerberg, the social network that immediately comes to mind isn't Facebook, but Twitter. Twitter has plenty of problems, but "only my crazy uncle uses it" isn't one of them. Twitter isn't unfashionable because of all the old people on it, at least not to nearly the same degree as Facebook.
Facebook's age demographic has changed beyond naturally ageing out, which might mean that it's possible it could change again in the other direction. I'm skeptical about that, but I don't think Abercrombie is a very good analogy. Fashion brands are famously cyclical; is the same true of social networks?