Good question, I have lived quite a transient life to date and have friends who move all over the world. I think it's all down to attitude really, I go for months without seeing my best friend (other than Charlotte) but our friendship has the strength to ignore the time scale. it's always as if it's only been yesterday that we have seen each other.
My Italian friends I see but once a year and again, it's like we have never spent time apart (except for more grey hairs) and my London buddies can't keep away from the boat!
And strangers? they are just friends that we haven't met yet....
(P.S) just about to send you a PM, have been using a touch screen to type of late and it's slowing me down!)