Just because someone uses Facebook too much doesn't mean that he/she is addicted. Get the concept right. Addiction, as for anything else, has a characteristic withdrawal reaction. What I mean is that a person who is addicted to Facebook will feel uncomfortable/irritated/depressed if he/she is forcefully denied access to Facebook.
There are people who post two status updates a day. The like and comment on all the posts. But force them to stop Facebook, they'll shift to some other activity. They are communicative, creative, enthusiastic and energetic. Their Facebook activity is just a reflection of these characteristics.
There are other people who never update their status. Rarely do they like or comment. They don't come online either. But deny them access to Facebook and their hands start itching.
So how to check if you are addicted?
Abstain from Facebook for 5 days. Check how many times you get the impulse to check for Facebook notifications. If the urge comes >5 times/day, you are addicted.
P.S. I did my 120 hour facebook abstinence test few months back and found I'm not addicted :)