Social networking is a close cousin of the other obsession of the moment: user-generated content. Of course, there is a difference. User-generated content is basically anything someone puts on the Web that is not created for overtly commercial purposes; it is often in response to something professionally created, or is derivative of it. So, it could be a blog, a message board, a homemade video on YouTube, or a customer’s book review on Amazon.com .

Social networking, on the other hand, is something potentially deeper — it represents a way to live one’s life online. In many ways, it is the two-dimensional version of what sites like Second Life aspire to be in 3-D: the digital you. And that ties to another earnestly overused term of art at the moment: engagement.

Engagement basically refers to the amount of time people spend doing one thing — reading a magazine, watching a TV show — but also to the depth of their participation.

Sony, for instance, paid $65 million for a video-sharing site called Grouper.com and started a nifty service through which you can load your favorite clip from one of its movies — say, Jack Nicholson barking, “You can’t handle the truth” at Tom Cruise in “A Few Good Men” — onto your MySpace or Facebook page.

Over the last few weeks, other media companies have accelerated their efforts in social networking. For example, the Hearst Corporation on January 8 bought a small company called eCrush.com. Many of the ventures sound like logical extensions of existing media brands because, afterall, media companies are all about attracting and keeping audiences and then figuring out ways to bring them closer to marketers.

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