People have an antiquated and two-dimensional view of psychological impetus. I used to as well.
What you are experiencing is not new and is not unique to you. The greatest minds are "the greatest" not because they were endowed with a magical ability to focus without effort, but because they learned how to manipulate their volition.
The other answers are okay but those solutions are, anecdotally, palliatives. The root of the issue in my opinion is: which part of my psyche am I feeding? You grow the psyche and brain potential through choice - deliberate choice not passive choice. The human body and brain have both evolved to prefer that which is comfortable, it is a survival imperative that is (sometimes) balanced by the equal imperative to diversify in order to discover more effective configurations.
The psyche and life are a little microcosm of evolution at work. When you choose that sugary food, that time wasting social network, that junk novel, or those friends that offer little valuable stimulation you are evolving a psychological state and neuronal pathways that keep making those choices easier and easier to make.
How do you get out of it? By making a choice. Given an array of choices within a given moment of time, picking the hardest of the choices will generally always serve you. In the kitchen for breakfast? Deciding to pick eggs and bacon over fruity pebbles may be "harder" if you've been programming yourself to pick quick, sugary, and easy foods over more nutritious but difficult to make food!
Remember all of it is relative, what is a hard choice now may be an easy choice a month from now if you've been "training" that generation of psychological states within you religiously.
Often, this discussion always ends up on personal morals because determining what's hard vs. easy is subjective. It also often lines up with personal meaning, beliefs, and a lot of other complicated factors that influence psychology and decision making.
Realize that you cannot move forward as an agent of evolution without choice and that in an array of choices you have the potential for causing an adaptive response in the physical organism that is your body, and/or in the psychogenic organism that is your psyche - which path that adaptation takes is up to the choices you make in every conscious moment of your life. Right down to which thoughts you choose to think (you do have a choice), which foods you choose to eat, which people you choose to socialize with, which activities you choose to engage in, &c...
Hope this helps. Blocking Social Networks or time-wasting sites is definitely useful, but it needs to be done within the greater context of why do I want to make these other choices. Without knowing why, it makes it very difficult for you to subjectively rank which choices will serve you more or less.
On that note, I'm going to choose to get back to work now :D