Luckily nowadays I'm coming back every time. But about leaving in general, it happens to quite many people. They join, they start, they get disappointed when their expectations aren't met or they're not enjoying their stay in here.
Quite many just leave. I'm not sure how many come back after the original disappointment.
But we could tell ourselves that all the best ones come back? :P As we all can see where I and we all have got so far!