Internet procrastination can have multiple causes, which aligned with each other provide the disastrous results.
random reinforcement schedule -- I have found also many valuable things online; the problem is I get them randomly
social activity superstimulus -- internet is to human interaction like sugar is to food
trivial inconveniences -- clicking another link in browser is easier than standing up and doing something else
instant feedback loop -- loading a webpage takes a second on average, many our activities are a few magnitudes of order slower
...and this is just for websites not optimized for being addictive. For example blogs are more addictive, because their timeline creates a sense of urgency. Blog comments create a lot of noise attached to the interesting content. Wikis contain too many links.
Now where's the scientific research on how to counter these effects?
Perhaps this is already well known, but it occurred to me yesterday and I thought I'd share it. The Internet seems particularly virulent as a form of procrastination; indeed, if, say, chatting at watercoolers took up as much time in the average office worker's day, we wouldn't make jokes about it. What is the feature that makes it so deadly? I suggest that it is the random reinforcement schedule: Every five minutes you "press the lever", that is, check forum X or site Y. And every six or seven checks you get the reward: Someone posted something interesting! This random reinforcement is ideal for creating addiction; thus, for example, slot machines.
As a way to avoid this effect, I'm going to strive not to do anything on the interwebs except at precisely defined times, or unless I have a specific goal in mind, say "Look up this method signature". Wish me luck, or better still, wish me willpower. :)