I’ve been wondering how much this is true versus it’s just how it has been getting older for most people for a longtime.

I once heard that Meetup is "one night stand for friends"

You're on the money. As I've elucidated elsewhere I think people are going about this backwards, because it's all we know: create organizations surrounding particular subjects, then have people join in. In many cases people are surfing around these groups as a means to an end, they don't care the slightest about the subject but they need it as a medium to connect with others. Not only is this contrived, it can lead to friction between individuals who want to focus on the subject and others who want to go on tangents away from it. This problem disappears if you simply have a space where people can choose to mingle or focus on their projects. There are collaborative workspaces, makerspaces, University grounds.. the issue is it can cost a pretty penny to rent, or attend, those spaces.

I've made friends with people at Meetup but I also agree with the above. My guess is that there are people that are looking to make friends, but whether they succeed or fail, they're going to stop going to the Meetup. What this means is that you're more likely to meet the regulars that really aren't that interested in making general friends.

That has exactly been my experience too.