Timo wrote this on the subject:
The shortening of attention span for Internet users is caused by dopamine addiction.
Discovering new information, i.e. an interesting site/page, gives a "rush" caused by the release of dopamine, the body's feel good chemical. Over time, you get desensitised to this "euphoric" effect, making you want to experience it more often. Clicking your mouse then effectively becomes a dopamine release switch.
Except when you click only to find spam or some otherwise uninteresting information. This results in withdrawal symptoms, making you want to click your mouse even more in order to get your fix.
So, if, upon discovering and interesting page, you find that you end up reading only the ingress of it, and then already feel the need to search for some other interesting information, you are addicted.
The key here is that the dopamine rush happens at the point of discovery. Not when actually consuming the information.