What's the difference between social media and a community?

At The Social Organization, Rachel Happe takes a look at the difference between building a social media site and a social media community. Social media, such as allowing comments to limited conversation around the content, creates communication, but rarely leads to a community of people with the same interests.
Happe cites these as the characteristics for social community: they're continuous, members gather around a common goal, communities take on various conversations lead by different leaders, and different leaders emerge over time.
Would you add any characteristics to this list?