Media is something that media companies control, and media is overwhelmingly one-way. The online social world is about as two-way, multi-way, any-way as it can be. Nobody controls it, not even Facebook, which found it can't even change its own terms of service.
Media is something people spend time with. So are online social interactions. That's a pretty tenuous reason to call it media. And while, as in media, you can advertise in social network sites, that is the least interesting use for them.
Here are some words you can use to think more clearly.
If you want to refer to the whole world of people connecting and drawing strength from each other online, you can call it the "social web" or the "social internet" (or you can call it the "groundswell," if you wish). It includes huge sites such as MySpace, communities, YouTube, the blogosphere and so on. (You could call the whole thing "Web 2.0," but people often use that term to refer to a set of technologies -- not the best way for advertisers to focus -- and it doesn't get directly at the people-to-people aspects.)
If you want to build an environment where consumers or other customers connect with you and each other, call it a "social application." It could be a community, a user-generated-content site, or even adding ratings and reviews to your site. By calling these applications, you remind yourself that 1) it's going to take some effort to build it right, and 2) people will interact with it. And you may even remind yourself that 3) it could last a long time, rather than coming and going quickly as advertising campaigns and media do.
If you're going to participate in a big social site (Facebook, MySpace, Twitter, YouTube), call it a "social-network site" (or just a "social network," for short). And you're often better off with a channel or a profile or an identity than an ad in such an environment.
But no matter what you do, the sooner you stop thinking of the social web as media, the better off you are.
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Josh Bernoff is the co-author of "Groundswell: Winning in a World Transformed by Social Technologies," a comprehensive analysis of corporate strategy for dealing with social technologies like blogs, social networks and wikis, and is a VP-principal analyst at Forrester Research. He blogs at blogs.forrester.com/groundswell.