Socializing is not safe for employees
The other stifling aspect of social networking at work is the Big Brother effect. Putting aside the kind of naïveté some employees may have about appropriate sharing at work (as well as on their personal social sites), there's the stark reality that whatever you post on a business social network is viewable, discoverable, and storable by your company. How safe does that make you feel?
If you say something to a colleague in the lunchroom or on the phone, you have some deniability if you crossed some line -- and certainly some forgettability. If you say something in a conference that you end up regretting, there's at least some context -- an outburst in a tense budget session is more understandable and forgivable than someone going postal in a routine status meeting.
You might remember the fear you experienced as a youngster about an infraction going into your "permanent file" at school or your first job. By your mid-20s, you realized that what went in rarely came out. No one went back into dusty archives to peruse the contents of your file, unless it involved a major HR or legal issue. The same is true of what you say in email or on a social network -- but because they store everything digitally, everything you write is much more easily discoverable through simple searches. Anyone looking for a reason to deny you a promotion or lay you off can much more easily find some excuse.
In the early years of corporate email, we saw lots of people fail to understand this detail, expressing sentiments that ended up harming them or their companies. (It still happens, in fact!) In the context of business social networking, where the environment is supposed to be very informal and personal, expect people to either forget they're being recorded or, worse for the desired collaboration, remember they're being recorded and thus not collaborate freely or honestly. (InfoWorld's Lisa Schmeiser has put together a nice guide to social networking business etiquette.)
Certainly, some business cultures don't penalize honesty and debate, as long as there is respect -- they'll probably avoid the Big Brother effect. I'm willing to bet that's a small minority. For the rest, honest and open communications in an informal environment is viewed as a trap set by the management police. You need to work very hard with management and employees to get past that fear.
External social marketing should be treated like any marketing
As for the other major use of social networking -- external marketing and customer listening -- you treat these tools like any other: The people who represent you professionally need training on what is permitted to say and do publicly. Too many companies want it both ways, asking employees to use their personal networks to further the firm's interests, but fearing that an employees' personal comments in a nonwork social media setting might harm the business.
You can't double dip; that's unsafe for both the company and its employees. Your media policies -- who can represent the company and under what circumstances -- need to apply to social networks. On one extreme is Apple, which forbids employees from discussing anything about Apple or its business in any forum. At the other extreme would be a company that decides the risk of an employee harming its reputation is small compared to letting the employee be public in a merged business/personal venue, such as a blog or a Twitter account.
Either way, the key is consisency of policy regardless of the technological medium. For some reason, many companies don't understand that, treating communications via social networking as wholly independent of all other communications channels, such as press interviews, conference presentations, and so on.