Do you use Facebook?
While being a Facebook user isn't a requirement of this position, the interviewer nonetheless wants to know if you actively use the site. Obviously, being a user indicates you have more familiarity with Facebook than someone who is not a user.
Be honest, as the interviewer has the ability to verify your answer. Simply explain if you use Facebook or not, and if so, for how long, how regularly, and in what capacity (personal, work, entertainment, communication, etc.).
"I've had a Facebook account for around 10 years now, and I use it every day. I love seeing pictures of my friends from college and my younger nieces and nephews. My favorite thing about Facebook though is being a part of several interest groups. Being able to connect with other people who care about the same causes I do is amazing and something that wouldn't have been easily possible before Facebook came along."
