Social Users

Picture found on Brian’s blog.

“The fact that Twitter is not meant as a “destination site,” might also play a role in the willingness for more off-site engagement because users may not feel as bothered by intrusive ads as they might on other social networks that offer more engrossing activities and require more concentration.” from MarketingCharts.

It is fashionable to conclude that social networks can be divided into 2 categories.

The Used Social Networks (USN).
This group allows these users to stay longer on its pages, use many features and enjoy the social aspect of the service. We can arrange in this group networks Facebook and LinkedIn for example. 
The Broadcast Social Networks (BSN).
These social networks are used to disseminate information, users preferred aspect receiving alerts and information and dissemination of messages (more or less massive) bound to a targeted audience. This group is particularly Twitter and Yahoo Meme.

The majority of sites or traditional media that add social features may be stored in the BSN group, their primary intention are typically to attract traffic to their pages by offering their users a means to broadcast information over the web. The best example I think is Youtube.