In thinking about friendship, and a few other things, I've come to realize that there are 3 basic needs for a friendship (as far as I'm concerned):
- Some form of communication, whether that's in-person, over the phone, through email or IM
- Shared memories and activities
- Trust
Without all three, a friendship will die.
You can try to keep it going, but once the first two are no more it's almost impossible to maintain. When the third goes, it will take a major effort on both sides to rebuild.