Another factor could be knowing that sometimes the people you used to know and see at school or work tend to leave (or you leave them) and not likely to see them much again after that. I have seen a few old schoolmates and occasionally spot someone from my past, but the majority of people I knew I have lost contact with.
With family, if someone goes on a trip or is away for a time, I don't seem to feel unduly worried (but still hope they remain safe). Similarly with internet friends, as long as I think they are okay, I don't have to worry that they haven't made much contact or updated their blog for a while (I just have to assume that they are busy, and 'no news is good news').
I don't want to think that 'familiarity breeds contempt' but maybe I just haven't found someone that I miss when they are not around.