Social networking addiction is a phrase sometimes used to refer to someone spending too much time using Facebook, Twitter and other forms of social media -- so much so that it interferes with other aspects of daily life.
A social media addict could be considered someone with a compulsion to use social media to excess -- constantly checking Facebook status updates or "stalking" people's profiles on Facebook, for example, for hours on end.
But it's hard to tell when fondness for an activity becomes a dependency and crosses the line into a damaging habit or addiction. Does spending three hours a day on Twitter reading random tweets from strangers mean you're addicted to Twitter? How about five hours? You could argue you were just reading headline news or needed to stay current in your field for work, right?