It's easy to get distracted by Facebook, among other things. Unless you delete your account (which feels a bit extreme to me), it's impossible to avoid spending more time than anticipated creeping around on social media accounts. One of the better ways to manage your time is to anticipate craving for spending time on these sites, and plan for it throughout your day. Just make sure you only spend enough time on there to get your 'fix', then move along.
http://lifehacker.com/5954148/beat-procrastination-and-distractions-by-scheduling-time-for-them?tag=productivity
http://lifehacker.com/5954148/beat-procrastination-and-distractions-by-scheduling-time-for-them?tag=productivity