It’s easy to imagine the difference, but to actually put it down into words has proven a little more challenging, but here it goes:
Online Communities have been around for way longer than Social Networks. Online communities are message boards or platforms that group together with one common interest of goal. Some common examples include youtube, and flickr, nested communities of video and photography enthusiasts respectively. Wikipedia is another good example of an online community.
Basically, online communities are held together by a common interest. It may be a certain hobby or something the members are passionate about. It could be a common goal, project, or merely the similar lifestyle choices, geographical locations, or professions.
When someone says ‘Social Network’, the first thing that probably comes to mind is Facebook (Thank you makers of the movie ‘The Social Network’!). Facebook, MySpace, and LinkedIn are the most popular social networks out there. Social networks are joined so that people can maintain their current relationships with their friends and family, get in touch with long lost friends, keep in touch, and stay in touch with said friends and family.
The one major visible difference between Social Networks and Online Communities lies in the fact that in social networking, people usually know the people they are connected to in person, whether as friends, family, colleagues etc… In Online Communities, it could be a gathering of strangers with one common goal/interest connecting them, such as Digg, a place for people to discover and share all sorts of interesting news links from all over the world.