Do you know what FRANCE stands for? - friendship remains and never can end. It was one of the acronyms used by schoolgirls in the olden days. That time there were only real live friends, no Facebook friends.
For that to happen, friendship needs to be nurtured. If you have no contact with each other for a long time, the bond will weaken. I also believe you should not take friends for granted. For example, a word or a gesture of thanks is not redundant when your friend has helped you.
I have a few long-time friends who still send me hard copy of greeting cards with scribbled messages and delivered by the postman. I think it is a case of old habits die hard. Apologetically, I did not send them back any cards.
Here is one from someone who is very proud of the motto of his Alma mater.