I manage to have the same social life I've ever had, without Facebook. I can't be an outlier. I mean, really, you have no way of seeing what your friends are doing without logging in Facebook? No one can call/text/email/stop over to tell you? That's hard to imagine.
Maybe there's a specific demographic that is affected in this manner?
I've had trouble with missing out on social events because of not being on Facebook -- mostly party invitations (like birthday or housewarming parties) and cases where people are going out with a small group to see a movie, concert, etc. They could e-mail or text me, but they generally don't. So I still get one-on-one invitations to dinner and so on, but I think a lot fewer large and small group invitations.
I think even one-on-one invitations may be affected somewhat because people may be reminded that they want to see people by interacting with them on social media. "Oh, there's an update from x; we should get dinner sometime, shouldn't we?"
A single friend can call/text/drop in to tell me about something, but what if that friend wanted to lets say inform a 100 of his contacts of this without having to send 100 texts/call/emails and keeping track of all of them for responses?
And social networks aren't just restricted to people but there are plenty of groups, organisations and clubs that want to reach out to not only all their members but potential audience too. Of course they can publish articles/ads in papers/magazines/TV, stick-up posters, send unsolicited email/calls etc., or they can have a Page/Group on Facebook.
I think that the key lies in forgetsusername's precise wording (https://news.ycombinator.com/item?id=11888033): "I manage to have the same social life I've ever had"—not necessarily the same one you've ever had. I, too, have the same social life that I've ever had without using Facebook. My answer to how I keep up with the plans that my friend broadcasts to 100 people only through Facebook is that I don't, and never have, participated in events that require notifications of 100 people. If I want to hang out with my friend, I do so; if my friend wants to host an enormous party, then I will wish him or her good luck, and see him or her later when we can have some time to ourselves.
(This means that my social life is probably pretty dull by the standards of those who participate in those parties, which is fine by me!)
Future demographic(s) will be impacted, due to not-knowing and/or not caring about it