What you talk about here is not “letting go” in the true sense. Distance can cause people to part and people you used to sit next to, they suddenly are miles away, and in a few years, may become strangers. Yes, social media or snail mail or mediums of communication help you stay in touch — so you hold on to them, the memories. The bond changes because of distance.
Try “letting go” of people while you still sit next to them at work or maybe pass by their house. At one time, you were there 24X7, now you cross ways and you can’t even say “Hi”…they are strangers, so close and yet so far.