Perhaps more than half of all construction organisations prohibit employee access to social media sites such as Facebook, YouTube and Twitter.
I think such blanket bans are often overkill. Far better, surely, to check when employees are using such sites, and how much, and tackle any perceived abuse on a case by case basis.
The whole blog post is now at http://www.blog.pwcom.co.uk/2009/10/02/why-social-media-bans-wont-work/