The problem with many addictions is that it can be hard to tell when a hobby has become more than just that, and taken a hold on you. It can also be hard to be honest with yourself when facing a list of symptoms, so make the extra effort now - we're going to go through a few.
1. You spend more time with the computer than with people. Doesn't matter if they are your family, your friends or random people you stalk at the mall - the first symptom of the problem is spending more time with the hunk of metal on your desk than with the people in your life.
If there are no longer any people in your life because you spend so much time online, then you really need to close your browser and book an appointment with a professional! A good start would be to use the paper version of the phone book.