While I was tempted to address multiple points, this is the one that really got my attention. I don't see how simply granting access will provide those skills. My Twitter experiences have shown that I haven't been able to get any value out of the service on my own. The web is full of anecdotal evidence of people regretting the personal information they inadvertently exposed on the web. Do you want them to use the same untrained behaviors in the company's name, with company information? Without training, those users could be more dangerous than helpful.

The early poll results indicate I'm in the minority, but I still view social networking as business tools primarily for the HR and Marketing departments only. Most of the other advantages Jack mentions can be handled in house, on this side of the firewall.