As I understand it, „loneliness” in the article is understood as the feeling of loneliness, not a measure of how sociable you are. You can be lonely while continually being around people which you have relationships with which look warm on the surface, but don’t give you much of a satisfaction. I think it is the case with a lot of people suffering from depression.
Note that our feelings are influenced greatly by social comparisons. For example, those whose immediate neighbors are all poorer than they feel much happier than those who are richer but the poorest in their immediate neighborhood. As Buddha said, expectations tend to rule feelings.
Modern society, both for dating and friendships, sets up false expectations ("The Illusion of Choice"), that often leave us without partners. Some combination of finding a crowd you like better, and lowering your expectations has to suffice: or nothing will.
Meanwhile men went to work where people have no choice but to communicate with you. You don't have to put effort into relationship, you are guaranteed not to be really lonely. In fact, too much relationship can be detrimental as you all compete for position and salary etc. Being a bit of jerk can pay of and you don't have to care whether people like you - if they don't it is up to them to deal with it. Plus there was influence of past war on expectations on males (be like veterans are).
That leads to two different kind of socializations for boys and girls - where boy socialization leads to loneliness in long term, when you are too old to pick up new habits.
Grandparents generation lived through world war, end of great migration of blacks, were adult through civil rights fights and had TV. American men went over the sea and even Easter European existing close knit village were destroyed by fighting, hunger, repressions, just to be later disrupt by communism.
As it happens, one set of my grandparents lived on a farm with no neighbors within something like four miles; the other, in a few small farming villages with maybe 32 people.
2) To a significant extent, it's possible to translate "I'm lonely" into "I'm too disgusted with the ethics, crudity, vile political opinions, manners, dress, accents, musical tastes, spiritual nonsense, etc of the people I meet these days to go out of my way to meet them again, thanks." This may be a downside to diversity (for those too-easily put off), or a reflection of the fact that we're all social climbers, who are too optimistic about our "social value" in the eyes of all the other social climbers, I don't know.
After all, almost all of us have solutions to loneliness handy if we aren't at all picky about our company. And back in the day, you couldn't be - your village was your world.
A hidden factor, IMHO, is the rapidity of cultural change. Last long enough (age 28 in my case) and you are necessarily a time-traveller, in a foreign culture to a large extent, and it hurts mightily to know that your tribe has moved on; but it has (unless you joined an uber-traditional church or cult as a youth.) If you opt to deny this, as many do, your world is gonna shrink and keep shrinking from that point on. You're gonna miss a lot of fun, too.
PS - volunteer.
PPS - I'm not 28, that's just how old I was when I realized I wasn't part of "the younger generation" anymore, and had to decide fast whether my generations' taste in everything was going to be forever supreme in my head, or to start making an effort to see what the real kids were up to, and whether I might like any of that, too. I chose the latter and bless that day.