Just found a great article on the CIO site, which details common mistakes companies make when building online communities. Money quotes:

• "Actually, the problem here is that the company generally doesn't recognize that they're in a conversation. They think of the online community as part of a 'marketing message,' which has a defined target audience, on-point theme, and so on. All of which can and should be controlled by a marketing department. And which completely misses the point of community."
• "Companies sometimes imagine they're using social networks or online communities to engage their audience. But the scope of their community involvement is to put the software in place and then, on an irregular basis, send in a well-meaning but powerless marketing person to make some sort of pronouncement."
• "If you really want to reach your community, and you truly mean to have a conversation that benefits both your firm and the people you intend to serve, then your executives should participate at least occasionally."

Further reading: 6 Stupid Mistakes Companies Make with Their Online Communities