On a Mac, you click on the battery icon in the Magic Menu. Select Energy Saver Preferences and then slide the Display Sleep slider to the desired time interval. On a PC, open up the Control Panel, select Hardware and Sound, then Power Options. Click 'Choose when to turn off the display' and select the time interval.
Many people feel the need to check up on these types of sites and apps, almost constantly, which can be problematic when you're trying to get things done from home. In workplaces, some employers block Twitter and Facebook sites just because of this fact. Setting up your computer this way allows you to do something akin to this, without being too drastic.Naturally, you can simply switch off those apps or simply close those tabs, but it's nice to stay informed about what's going on. This trick will also work with multi-monitor setups, in which you can customize each monitor's sleep settings. If you don't have a desktop, you can try using an older laptop. We've done this before, and if you've got enough space, it works fine.
Also, depending on your work situation, you might have to keep an IM client, a tab, Twitter, or Facebook open to communicate with colleagues, partners, and sources. It's an easy way of staying productive, no matter what is happening with your social networking circle.
(photos by Range)