There's no simple answer to these questions. It's healthy when your f2f life is integrated with your cyberlife. Their cyberspace activity becomes a world unto itself. They don't talk about it with the people in their f2f life. It becomes a walled-off substitute or escape from their life.
Reality testing is lost.
Fixing this dissociation is an implicit or explicit component of many of the techniques for helping internet addicted people. On the other hand, healthy internet use means integrating the f2f and cyberspace worlds. You talk about your online life with your real world family and friends. You bring your real identity, interests, and skills into your online community. You call on the phone or meet in-person the people you know online.