The first step is to ask your employees (and yourself) to separate their business or professional networking as much as possible from their personal networking. For example, discourage the use of company email, IM, Wikis, etc. for anything but conducting professional-quality company business. If they want to send jokes or other private communications, ask them to use their personal network accounts, and on their own devices or terminals as well. It is a good idea to remind employees that the company"owns" the business digital equipment and accounts, and has the right to monitor anything sent or received on these devices or accounts!
Secondly, remind your employees of their agreement to not disclose company confidential information. Most companies include organizational details like reporting relationships, job descriptions and conversations between employees as confidential; so bad-mouthing your boss on your Facebook page or Tweeting about your dysfunctional co-workers would violate this agreement. A particularly egregious post or tweet could lead to dismissal or even a lawsuit. We all know by now that anything one posts on any digital network can show up anywhere else on the internet at any time in the future.
Finally, in addition to clarifying the boundaries, you can provide incentives for courteous and professional behavior while networking on behalf of the company internally or externally. Incentives can be in the form of explicit remarks on Performance Evaluations, peer recognition, or even bonuses.
Unless it is very obscure, there will be (and probably are) plenty of postings about your business in the social media. The idea is not to avoid or to try to control this exposure; it is to have the posting reflect the true value and integrity of your culture and your brand. There are a growing number of tools you can use - such as Radian6 - to monitor the buzz and see what both employees and outsiders are posting about your company.
Social media are here and growing. As business leaders we cannot control them, but we can learn to live with them symbiotically.