Slightly political rant with which I fervently agree: Block Porn? Don't Bother. As an aside, I think Chris's arguments are also highly relevant for organizations looking not to block porn but social media:

What makes you think that applying a filter, which can be turned off [note: or blocked at the firewall, which can be circumvented using employee-owned devices], will change this? If Mummy or Daddy your employees want to see naughty-naughty access Facebook on the computer work network, they will turn the filter off...

In other words, it's a training and governance issue, not a block it issue. And if employees are accessing Facebook for non-work-related things and wasting organizational time and resources, that's a management and employee thing, not a Facebook thing.

Anyway, read the whole thing at Chris's blog.