What makes a good community website for museums?

With the rise of social media and community sites (such as Facebook, Flickr and an ocean of blogs), I've been wondering: what factors lead to a "successful" community site? How can we tell if a museum community site is working? Here are a few things that come to mind:

• Quality of visitor postings.  Not just "that's a great photo!", but responses that evoke a sense of the person who is writing, that he or she is fully engaged in the topic.
• Sizable number of participants. "Community" implies number, and although it's arbitrary, it seems you'd want at least 10-15 people regularly responding and posting; could well be in the hundreds for a larger site.
• Multiple authors. If a museum has just one author, potential for community is limited to a particular voice and vulnerable to staff changes.
• Structure for participation. Does the site have a clarity and appeal that draws you in and makes you want to participate?
• Impact. Is there evidence that participants do anything differently outside the forum as a result? (My thanks to Bruce Wyman for flagging this). Museums are increasingly called to add value to their communities at large -- do they use online community to support this? At minimum, do they encourage new experiences?

What have I missed? Any other thoughts?