Yes, it proves that network effects can force people into bad equilibra. If you think high university costs are harmful for society in the long run, but if you personally don't attend you will incur significant cost in the short run, with nobody cooperating, how do you behave?
Not really - I would love to be able to delete my account from Facebook but it's the only place some people that I want to keep in touch with will use.
(I did the next best thing - deleted the apps and only check the website infrequently.)
I am claiming your statement "and they feel perfectly comfortable" is unknown. Further that it is highly unlikely.
Many is not most - it probably is most of my friends and relatives, but they are representative of nothing at all. Yet there is now a certain discomfort in almost all discussions of Facebook - no matter where that takes place - that simply was not there 5 or so years ago.
Attendance absolutely proves something when it comes to social media. You can't make a comparison between social media and food because one is necessary for survival and one isn't (I'm sure I'm going to invite a ton of comments here on how 'Facebook has become so ubiquitous and powerful it's now synonymous with survival!') If the negatives really outweighed the positives people would be leaving Facebook by the droves. It's only on HN and in the news media that this narrative is being spun.