I think it's absolutely true, though not all friendships. But most of the time, there comes a point when one can't seem to react, take or give more and the other realises it and feels the same (or sometimes not) and that's when you decide to accept and part ways on good terms...as Arlene said.
I realized this quite early in my life when my best friend moved to another country and despite many many attempts later, I was unable to establish any sort of friendship...things just never clicked anymore. I think this happens if one over-generalizes the relationship and unintentionally begins to look beyond its mere simplicity.
Ofcourse this doesn't apply to everyone...some have that disposition or the right attitude towards friendship that enables them to maintain long-lasting relationships (either online or otherwise).
Every friendship and every one of those people you've let go (or who left you) are all memorable in their own way.