Feels as though I need a dictionary for my dictionary now that so many words are taking on new, techno-based meanings.
You have heard about the Twitter explosion over the last year. You probably have a Facebook page. You may even have a MySpace page, DIGG account, or bookmarks on Delicious. If you have those connections, your employees, most of them, anyway, probably do as well.
The social media push may have you examining your handbook for policy update needs. Your electronic media policy may be so broad that you're already covered. If you're not, though, hold tight before crafting an entirely separate policy. A tack-on to a computer/equipment use policy can address any privacy expectations (there are very few legitimate ones in the workplace). A sentence to your anti-harassment/-discrimination policy can cover the social media outlets.
In short, keep it short. No need to add another page to your handbook over sites meant to break communication down into 140 words or less.