I'm amazed at how much energy and time people put into Facebook. Definitely productivity impact at work. Companies might consider blocking for that reason alone (not mentioning the security risks for employees).
I said it before on this site, and I say it again here: we are giving the likes of Facebook too much power over our lives. Such social media should be run as non-for-profit organisations a la Wikipedia, in my opinion. We need many of such sites with standard interfaces between them allowing for seemless integration. I believe this will happen in time.
PS. I predict Facebook's demise in the not-too-distant future, in part because of security concerns.
It's not that difficult to be irresponsible with personal information via many different vehicles. I can lose my wallet, lose my phone, not shred sensitive documents, etc. I can also be more conscientious and be responsible with that data.
The biggest problem with these Internet systems like Facebook isn't that they can expose personal data. It's that they can expose it to such a broad audience so rapidly. Some of the functionality requires more exposure than other parts and we just have to weigh the risk vs. reward. There are no guarantees anywhere, but with care and understanding the risk, Facebook is just another modern-world communication tool.