The ubiquitous Facebook status update. The reason I got hooked on the service in the first place.
The beauty of the Status update (a very little thing and a very BIG reason people are so addicted to Facebook) is its digestibility. You don't need to have long catch up conversations with you friends anymore, you're always caught up. You know what they're up to, when they're happy, when they're sad and can react immediately to the situation.
Without the status update Facebook would be a place you went every so often to see if anyone had new silly pictures of themselves posted. With the status update, you're constantly returning to find out what's new with everyone you know.
It feeds into the voyeuristic nature of our society and keeps you coming back for more.