Are you an information addict?
kees: A long time ago I turned from effective use of the Internet into a consuming mood. Consuming information.  “Enough is enough” doesn't play any role on the Internet. You will never consume enough information and there's always something new to discover. Most of the time the information you read will not add anything to your knowledge, or your productivity. Don't blame the Internet, understand that it is all your fault. If you are a  person who likes to procrastinate, prefer to read than to act, you will easily fell in the traps of the Internets. Personally these are my solutions to handle the problem;If you think something is an interesting read, skim it(or in other words, scan it for useful information), bookmark it for later. And probably you will never spent a lot of time on the article . And if a need for this article pops in your head, you can easily find it back.Never use on-line documentation for studying or as a reference when doing something productive on the computer. Always download the necessary files (pdfs online) beforehand. 
Prepare you for your task, collect the information on line but when starting the task: just switch of the Internet, even better don't have any physical connection with the web.I love a minimalistic desktop, remove the webbrowsers' icons on your desktop or any menu panel. Just start your browser with a keyboard shortcut. The inviting sweet internet entrances, which you're conditioned to press when you want to avoid a task or need an information kick are gone. Recondition yourself when using the new shortcuts: Think about the purpose you want to use the web and what time you want to spent on that task.You never spent time a lot of time consuming useful information, because you will directly apply this information to anything productive. With the Internet, you find the stuff you want in a glance. The problem is information you're consuming, the interesting stuff you want to read, all the stuff what's coming in an endless stream.Don't read any opinionated stuff published on an obscure web blog written by an obscure person without any (productive) accomplishments. Probably you won't miss anything useful.Just check your favorite websites daily, preferably weekly. Try to optimize the interval between visits. If you follow some interesting people or blogs, which doesn't publish daily, just subscribe to the feed using your email.Allow yourself time on the Internet to do and read anything you like,after work and just set yourself a time limit. Because overall for me the internet is a great discovery mechanism, which increases my productivity. But do it consciously.Most of the time I don't use the Internet dominantly when I'm sitting beyond my screen. I always need it for something I want to do.PS I only use Facebook is an interactive address book, twitter to follow some interesting persons and Gtalk/AOl for colleagues, when working on a project.