The Null Device
One useful feature which Facebook, the social network site of the moment, lacks is the ability to compartmentalise information. Whereas on LiveJournal you can define filters and make posts visible to only some of them, on Facebook, every piece of information you published is visible to all your contacts. (Except for those who can only see a limited profile, who are forever stuck in a purgatory of sort-of being "friends" with you whilst being left out of all the fun.)
Being able to compartmentalise your information is useful; there are undoubtedly things you want to tell some of your friends whilst not letting the rest know, other things you're happy sharing with a different (though possibly overlapping) subset, and others you're happy letting anyone know. Think, for example, of talking about work without pissing off coworkers, or confiding about your lovelife, or discussing health issues without overwhelming others with "too much information". As social software becomes an integral part of the social support networks of today's compulsively multitasking, digitally connected population, such controls become more a necessity than a luxury.
Fortunately, Facebook's users have come up with a workaround: creatign members-only groups in lieu of privileged posts. So next time you see a group with an otherwise uncompelling name like "Emma has a new phone number", you'll know what's going on.
(via confused in calcutta)