If even 10% of my friends weren't there, I couldn't be planning events there for example. If you have to call or email a subset of people you might as well choose a different way of planning (e.g. invite people via email to a dedicated planning service).

So to me the value is that everyone I know is there. Not just 95% but 99%. So if even a fraction left, I could much easier just leave. My "social circle" on facebook could quickly just disappear, and I suspect this goes for lots of other people. Because the strength of facebook is that every social group is connected - but that will be how the cancer spreads too.