To anyone who’s worked in direct fundraising, “churn” isn’t exactly a new concept. Indeed, losing members of any list comes with the territory of appealing for money to support causes. Yet when users leave social networks it seems somehow different than opting out of an email list. That’s because the investment of personal time and informational capital is much higher than signing up for an e-blast. You’ve made “friends” or garnered “followers.” You’ve created an identity. You’re part of an interconnected network sharing not only your favorite causes, but your likes and dislikes, the books you’re reading, the music you like, the movies you love.

When someone signs off from Facebook – someone who’s been pretty active and involved – it feels like the person’s disappeared. When an active Twitterer leaves, there’s a void; a channel of information with a real person behind it has gone dark.

And then there’s the hype factor. Facebook and Twitter have been deservedly promoted as the largest (Facebook) and the coolest (Twitter) social networks ever launched, the new Microsoft and Apple, harbingers of vast societal change. Yet the inevitable “is that all there is?” factor was always heading down the highway. You knew there’d be a backlash, particularly against two private corporations that assumed such important societal positions. Virginia Heffernan’s column in last Sunday’s New York Times was the roadside flare for that head-on collision, taking on the anecdotal surge in Facebook farewells among the writer’s friends:

Things fall apart; the center cannot hold. Facebook, the online social grid, could not command loyalty forever. If you ask around, as I did, you’ll find quitters. One person shut down her account because she disliked how nosy it made her. Another thought the scene had turned desperate. A third feared stalkers. A fourth believed his privacy was compromised. A fifth disappeared without a word.

Ask around, and you’d find quitters on Twitter too. And among some of the next rung sites like Digg, Ning and Mahalo. And among bloggers, Flickr photo-sharers, YouTube videographers, and various people-powered networks of all shapes and sizes. Churn happens. Time is limited. Life intervenes. As Heffernan (I writer I admire) says later in the piece: “Many seem to have just lost their appetite for it: they just stopped wanting to look at other people’s photos and résumés and updates, or have their own subject to scrutiny.”

Exactly. Yet as my friend, venture capitalist Fred Wilson, responded on his blog: “…churn is part of online media, particularly social media. People come and go. Some stick around, some don’t. These stories about quitters are true of course, but they miss the big picture. More and more people are using these services every day.” And then Fred posts the latest Comscore numbers for Facebook and Twitter. About 52 million people visited to super-hyped Twitter in July, makiing it the 47th most popular site on the Web – an incredible growth story that continues. And an whopping 370 million visited Facebook. As Fred says: “Facebook is a global juggernaut. It is the fourth most popular website in the world after Google, Microsoft, and Yahoo!”

So if Aunt Sally and that boy you dated back in college drop from site on Facebook – and that social entrepreneur tires of Twitter – it’s interesting from a personal standpoint. But the sheer size and continued growth of the largest social media properties makes them ever more important on the social commons, in my view – particularly as they continue to be places of experimentation and innovation in fundraising and philanthropy.