Someone with whom you shared a friendship/relationship that simply drifted out of your life.
I have had a lot of people drift in and out of my life: lovers, friends, boyfriends. I know so many people now that I am older and I do my best to keep up with them. It's really hard as it is for me as a blogger when I host five different blogs along with running profiles on various social and bookmarking networks so that I can always get new readers, I can only really see the people whom I have been really close with for a while. In so far as keeping in touch, I am really thankful to social networks like Myspace (even though I'm never on there anymore), Facebook and Twitter because if I didn't have them I'd lose contact with a ton of people.
I do miss them, especially the ones who just stopped talking to me for no reason. I guess those hurt the most. I don't know what it would be like to just cut people out of my life like that and not even give the other person some closure.