In my old firm which was much larger then yes I did consider them friends, we would go out quite a lot together at nights. I switched to a smaller firm and I still meet my old friends for lunch.
My newer firm I consider them colleagues, I exchange texts with one of the girls during matches (we are both tennis fans) but other than that no. Perhaps because it is a smaller firm and also all but one is married and two have kids on the way!
We don't really have regular nights out, again because of the dynamics in the office but I organise things like bake offs and we do lunches and things. I do consider them friends but I have different levels of friends and their place in my life.
As you say, we spend a lot of time with people we work with, so it is important to get along, you don't want to spend the majority of your time in a hostile environment as it isn't healthy.