We have a very small family. I was always brought up calling my grandmother's close friends Aunty or Uncle even though they weren't related. They were the people that were there for my grandmother through thick and thin and I wouldn't dream of not treating them as if they were blood .
Not everyone has relationships like that though. We've had a couple of friends try to claim the title but we just subtly discouraged it by not using the it ourselves.
DS only has one Aunty that's not actually related, and that's my best friend. None of our other friends have been around in our lives for long enough to justify it.