We don't keep much from each other, I usually tell him most things and we discuss things when I need to talk about something I've heard. He tells me about his friends, I tell him about mine. My best friend is my oldest friend of 24 years (we were also neighbours) and when I tell her something, I do know there is a chance she will tell her dh. It doesn't bother me, it happens.
Dh and I have an unspoken agreement we are to never repeat what we tell each other though and we would never break each others trust, so telliing me something will never go as far as me or him. I even know which of his mates are having affairs, and he knows alot about my friends. In saying that he is a very non judgmental person and not much phases him.
But for me, sometimes I need to talk about things and he is the only one I can trust completely. So if a friend tells me about her affair or abortion, I need to talk about my feelings. Dh is a good person to do that with, he's not a friend that can break my trust and confidence, it's like talking to myself who has a different perspective. And it saves me going online to vent to strangers.