I think you're making some unfounded assumptions in your question.
what is your social life outside of academia like as a postdoc?
When you ask this question, you then immediately turned back to academia. Your social life as a postdoc is, in many ways, whatever you want it to be. For example, a great deal of my social life outside of academia was a local group devoted to my hobby.
To address a few other points you make:
- There may be fewer postdocs, but there is no reason your social life is restricted to this "tier". As I mentioned above, most of my social life was with non-academics, or at least non-academics in my particular lab/institute. I made friends with some of the faculty. With a few of the graduate students. With the research staff. There is no clubhouse with "Postdocs Only!" written in Sharpie that you need to rely on.
- "There are no excuses to spend a ton of time with one another" - You're assuming this is true, but it's not necessarily true. I will say that, in my own personal experience, the most intensive time I spent in contact with other researchers was indeed during my postdoc, and there were some friendships formed "in the trenches".
- "possibly have nothing in common to talk about in terms of academics." There is the commonality of being an academic. Insane PIs. Funding lines being universally horrible. Anxiety about finding a job afterward. Reviewer #2's asinine comments...
- "I do not have the luxury of choosing the people that I like anymore." Yes, you do. It's called making friends. They just don't have to be postdocs.
Literally the most social I have ever been in my life was as a postdoc.