This has varied over time. I've found that it's definitely more about "who you'd like to have a beer with" at early-to-mid stage startups, and becomes more about actually being someone you'll both be effective working alongside, as well as being someone you'll enjoy* working with, as the company matures.

*enjoy here means that you'll be able to enjoy a non-toxic work atmosphere, will be able to provide and receive constructive criticism and comments about your contributions; the healthy working relationship does sometimes lead to enjoying having a beer with them from time to time, but the former is definitely the primary goal in trying to find a culture fit.