Since I've run one of these "social" book clubs for a few years (we actually say "you're welcome to come if you haven't read the book" in our Meetup post), perhaps I can give some insight into what they're like. It's interesting to reflect on how things could be done better, and over the years I've spent a good amount of time doing that and making tweaks.
First of all, the group I'm in tends to read mostly non-fiction books about history, philosophy, science, social issues, and economics. I've never thought about it before, but this is perhaps one big reason why people can drop in and have a good discussion even if they didn't read the book. People have opinions about these subjects and personal experiences that are often interesting even if they didn't read "Evicted: Poverty and Profit in the American City" or "Why Buddhism is True".
That said, it can also get frustrating and annoying at times, particularly when people pile on with counter-arguments that the author has already carefully addressed. Then I sometimes find myself having to articulate a low-fidelity version of the author's argument and defend it, when the book obviously does a better version of that. I tend to see these experiences as just part of the price for this kind of book group.
We make other trade-offs as well. The group is advertised on Meetup and it's a part of a secular humanist organization I'm in, which means people come for all kinds of reasons. Maybe they saw the book and were intrigued by it, others are curious about or identify with humanism in general, occasionally people seem to be new to the US and want to practice their English and reading. These different groups are inconsistent in their attendance as you can imagine. There's a core group that came for whatever reason and got "drawn in" with the community and the types of discussions and come back most months. Then there are people who only come if they liked that particular book. Then there are people we'll never see again.
In other words, we have a higher churn rate than the "group of friends" or "high commitment" book clubs probably do, so we need to bring in more people. But this also has its advantages, because I meet all kinds of people in jobs, age groups, and parts of the bay area that I would never normally meet, and hear interesting stories from different walks of life.
Overall I think our club serves a good purpose - it gets people out of the house, meeting others, and learning something new from a book each month, even if we're not "sucking the marrow out" of the book and only getting a more superficial benefit.