Is facebook addictive?

So Friday night my sister, one of my friends, and me went to my little brothers football game. Before we left for the game we were at my friends house and they both checked their facebook account. How long does a football game last? I think this one was three hours. But when we got back to her house from the game they both ran to the computer to check their accounts on facebook, again. Seriously how many times a night do you need to check it? It's not like either of them had nothing else to do or were sitting at home bored. Maybe it is just something that I don't understand, but once a day, maybe even once every three days is plenty. Don't you think if it was something really important that needed to be discussed they could call you?