Dare Obasanjo - with lots of good linkage - explains why Walled Gardens (like Facebook) become popular. There's even an advantage to not having full external visibility - post stupid pictures (etc) from some collegiate party on MySpace, and you might find that hiring managers are a bit leery about you. Do it on Facebook, and it's invisible to anyone but your friends. I can definitely see that as being a huge thing for the current crop of high school and college age users of these networks.

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