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. Forming friendships in that situation is difficult because at this point many of those people have moved away for greener pastures.
Most of my friends are people I met trough my extracurricular social circles. Personally I like that almost all of my friends have different jobs in different fields. There's never work talk in social settings and I get a more diverse perspective on the world.