When to let go?

Is there ever a point in time when friendships have run their course? Should one eventually just call it a day and say "nice knowing you?" I have a lot of friendships that only exist based on memories. Does the time one has known someone overrule the fact that we have out grown any commonalities that once kept us together? At this point our talks resemble... "remember this...?" and "what have you been up to?". Part of me enjoys catching up and hearing what long lost friends have been up to, but at the same time, I find I am generally the one putting in any effort. Does on email a year sustain a friendship? Can memories alone make a friendship worth working for?