I could easily go days without thinking about reaching out to people. It's not that I dislike them or don't care, but that I often get wrapped up in things going on around me. I get home from work, find something to eat, and start playing a game, watching something online, or catching up on my feed reader, and that will very easily consume my entire evening until it's time to go to bed.
I've lost touch with quite a few people this way. They probably think I'm trying to ignore so they don't reach out. Most of the people I've stayed in touch with are those who I see at work or who have been persistent about contacting me regularly (a few times a month a few times a week).