Perhaps I’m the odd man out here, but I have met most of my friends at work.
One of my best friends is the girl who actually hired me several years ago at a restaurant; she was my manager for about six months, and then we both quit around the same time and have remained great friends since then.
One of my other best girlfriends was a coworker at a bar I worked at; she no longer works there and I just quit myself, but we are fast friends.
I am also great friends with two managers; one of them no longer works with me, and one of them (up until two days ago when I quit) was still my manager.
All of these people, managers included, were/are friends outside the workplace; we socialized frequently outside of work and generally had a great time while working. The only time it “affected” our working relationship, particularly with the managers, was when we disagreed on something work-related; at that point I would explain my point both as an employee and a friend. It never became personal.
And now that I don’t work there anymore, I fully expect to remain good friends with the manager I was still working with. (In fact, it will probably improve our friendship because it won’t be affected at all by work.)
However, I have worked with like billions of people, and only four of them emerged as good friends of mine…in general I am friendly with coworkers but I don’t go out of my way to socialize with them unless I actively like them. Mainly b/c I don’t like a lot of people. I see no point wasting time socializing with people I don’t really like, so I’m pretty particular about the friends I make at work.
But the friends I make at work, I keep.