Sports leagues, fraternal societies (like the Elks or Masons), parent-teach organizations, volunteer organizations (like the Boy Scouts or Red Cross), and labor unions have all seen their memberships steadily decline for the better part of the last 50 years.[1] Doubly so among the youngest generations.
Beyond that, in general we have much fewer close social ties than we used to. Marriage rates have plummeted[2], which means many fewer people have a spouse. Surveys show that Americans have only about half the number of close friends as they did a generation ago.[3] The sizable majority of Americans don't even know their neighbors.[4] And of course birth rates have plummeted, which means that the number of siblings the average person has, and eventually aunts, uncles and cousins are all decreasing.
In general the central sociological fact of the modern-era is the unprecedented degree of social atomization that most of us now face.
[1] https://en.wikipedia.org/wiki/Bowling_Alone [2] https://www.pewresearch.org/fact-tank/2017/09/14/as-u-s-marr... [3] https://www.barna.com/research/friends-loneliness/ [4] https://www.studyfinds.org/sign-of-the-times-75-of-adults-ar...