[TheEzEzz]

I could easily see this going the other way. Life long single people develop strong social networks that they keep investing in into old age. Married (and with children especially!) couples invest less time in their social network, in old age they then have many fewer friends when their children leave and their spouse passes.

(I'm not sure this is true or not, but seems plausible. I agree with the author that we should get better data to resolve these questions).

[AStonesThrow]

Marriage is sort of like adoption because you're given a new family. It's different from adoption because you get to keep your existing family.

Successful married couples are investing in an internal social network: offspring and extended family. If this network can meet their needs then they have less reason to go outside of it.

Childbearing families are going to encounter other parents and their social network will change accordingly. Play dates, day care, school parents and extracurricular groups will be in their orbit now.

Conversely, single people may rely on their own families for social support, and they may not need to go outside of that, either. Woe betide the single person whose family is unsupportive, because friends and acquaintances are a faint substitute. Such a person could develop ties with their employer, professional orgs, non-profits and community to stay sane and healthy. Or they don't--there are plenty of dysfunctional singles who don't need a marriage in order to suffer and fail at life.