
Robert Putnam, a sociologist from Harvard, is probably best known for his seminal book, Bowling Alone: The Collapse and Revival of American Community. From wikipedia:
Putnam surveys the decline of social capital in the United States since 1950. He has described the reduction in all the forms of in-person social intercourse upon which Americans used to found, educate, and enrich the fabric of their social lives. He argues that this undermines the active civil engagement which a strong democracy requires from its citizens.
Putnam outlines at length the waning of voluntary associations.
Putnam noted the aggregate loss in membership and number of volunteers in many existing civic organizations such as religious groups (Knights of Columbus, B’nai Brith, etc.), labor unions, parent–teacher associations, Federation of Women’s Clubs, League of Women Voters, military veterans’ organizations, volunteers with Boy Scouts and the Red Cross, and fraternal organizations (Lions Clubs, Benevolent and Protective Order of Elks, United States Junior Chamber, Freemasonry, Rotary, Kiwanis, etc.).[1] Putnam used bowling as an example to illustrate this; although the number of people who bowled had increased in the last 20 years, the number of people who bowled in leagues had decreased. If people bowled alone, they did not participate in the social interaction and civic discussions that might occur in a league environment.[1]
In my mind what is being tracked here are the platters where exchanges took place, and not the actual amount of work or resources engaged in production of a shared asset. The bowling leagues provided the venue for friends and neighbors to gather, extend a hand, overhear who needed a job, step-up if someone passed. But as the clubs of father’s or grandfather’s became out of date it is just as feasible that younger generations found new places to gather and organize.
But why then are the number of volunteer hours reported in the American Community Survey lower? Perhaps people were engaged in informal volunteer work, unpaid work to help a friend or member of the community, without being associated with a formal organization.
Consider American life from the 70’s up to the writing of this paper in 1995 (followed by the book in 2000).

The observations in the book coincide with two decades of increasing divorce rates in the US, of households being split apart. Moving homes is time consuming enough, then there are the additional logistics of separating both financially and emotionally when the cause is a divorce. This closing out of marital arrangements can easily last a year or more.
Furthermore the schedule of a single person with children is bound to be quite different from a married couple. Whereas there may have been an easy agreement of, ‘you get to golf, I get book club, you handle sports leagues, I’ll manage PTA,’ everything gets a little trickier to plan when you no longer share a household.
There are a variety of reasons why marriages came apart and people preferred to dedicate resources to undoing their marital bonds rather than hanging in for the golden anniversary. There was a reckoning with this institution that may still be in play. But the reality remains that there are only so many out-of-paid-work hours to devote to family and community. If those are being devoted to a reassessment of marriage, then they will not show up on the American Community Survey under volunteer hours.