You are missing a huge point. MySpace and Friendster didn't got out of fashion suddenly because social is not "sticky". Friendster was heavily burdened with scaling issues, design complexity and bugs that basically force moved users to FB which remained fast, slick and extremely usable. Similarly, MySpace just couldn't engineer features faster that users wanted. The big point here is that social is very hard engineering problem and FB excelled it in early days. They are currently failing, in my opinion, because their newsfeed algorithm shows too much content from groups and much less from friends. The group content is something people will like initially but becomes monotonic later and so they quit.

So, the social platforms have failed historically because of engineering issues that their competitior excelled at.