I've live on a cul-de-sac (back) in Extreme Flyover Country now for a whopping ... three months, and I met all the neighbors and chat with 'em often. People in stores are very open and will talk if spoken to.

I find driving my car is much more efficient than taking trains allowing me to make multiple stops thus talking with numerous people through out the day.

I have lived in my Manhattan building for only 2 years. Know most of the other residents, the owners of neighborhood restaurants, local politicians, et cetera.

That said, I do not feel that expectation of social and civic engagement in San Francisco.

Yup. I've lived in SF since 2000 and every time I go to NYC it's way easier to connect with people. It's either I'm different when I'm in NYC or it's SF. Or maybe a bit of both but your comment resonates very strongly with me.

It very much depends on the city and even the specific part of the city. e.g. London, despite mostly being as you describe, has areas where there is a strong sense of community, even for new comers. Typically more working class areas.

In Brazil, architects often build a social space into their apartments. It's a good idea and works really well, but the people have to want to socialise together in the first place.