Forgive me if I get the citation wrong, but I believe I remember listening to Ezra Klein talking how for much of the 40s/50s/60s, communities were organized by churches and community institutions. I would easily add military service to this as well.

If you were a doctor, you would still know your community fairly well; whether they be a farmer or another blue collar worker. As we've replaced the prominence of community based institutions such as churches with elite institutions (primarily colleges and workplaces), we've isolated lots of people who aren't able to participate in these communities (due to many inequalities). Obviously these previous community institutions had major flaws (racism/sexism), but they still provided a shared experience that makes a diverse and broad social group easier.