Oven Mitts

Id imagine that if you left some idle in your friends list for too long and don't talk to them, they would remove you. like any friendship it's important to maintain contact with people; whether that be by PM or simply responding to their status updates. If they just sit their idle, they may begin to feel like a notch in your friends list belt. This is just speculation though

Little activity?

If they don't want you as a friend they don't deserve you as a friend! Cliché I know, but you're awesome, there are plenty of people who care, so don't get too hung up on those who don't ^^