I once had a friend who I cared for very much. We both liked the friendship we had.

I was the talkative one. But he spoke too. And now, a few months later... He moved on to a new group of friends. Which I don't mind. As long as I had the same warmth.
But I didn't get the vibe from him anymore.
Today I brought up in conversation how our changed equation had changed.
He seems to have noticed it too.
I told him he needed to put in more into the friendship lest we become strangers.
Should I write it all in a letter and give it to him? My side of the story?
Or should I just watch my so called bond fade away before my eyes because I don't see him put an effort?