How so? I think eyeareque is correct.
In the days before the Internet, there were generally three ways to stay in touch with distant friends: writing letters, making telephone calls, or visiting in person. All three required varying degrees of time and effort on ones part, but this was offset by the simple pleasure of conversing with a friend. People didn't bother engaging in this sort of activity with acquaintances or "rather pleasant kinda-friends", because the personal connection didn't exist. There was no "payoff" significant enough to offset the effort.
The likes of Facebook remove most of this time and effort requirement. You can "friend" people you haven't spoken to in years or even decades with a minimum of difficulty. While I'm sure there are plenty of cases where the lack of communication could be due to external causes (divorce, reassignment, death, job loss, etc) there are undoubtedly many more cases where people simply "drifted apart" because they found they didn't have strong common interests.
Many Facebook (and similar social media) relationships really aren't "genuine" (as eyeareque stated) for this reason. Neither party really is interested in putting additional effort into the relationship beyond a simple "like" button click every now and then, if even that.
The OP's story is sad, and while I agree with their summary that we should treasure our friends and family while we have them, let's not pretend that the Golden Age of Social Media has somehow provided us with a revolutionary relationship tool.
Facebook is a dynamic, 21st Century version of the old "phone number and address book": full of people to whom you might send a Christmas card, but little else.