> Most of the people I work with are highly ambitious and globally mobile. Our average tenure is something like a year. People who have been at the company for two years are veterans.

That sounds like a highly irregular company. My last company, which wasn't very big (<100), had half a dozen people who had been there 15+ years. The majority had been there 3+. A bunch of us would go out for a group lunch nearly every Friday (not organized by management). We'd do group Halloween costumes and wear them to the office. People would have game nights at their houses and invite people over.

I realize that my anecdote is probably also outside of the norm, but I just wanted to illustrate how wide the range is.