Sites with multiple authors

Before taking any decision on the values you will provide for fig.1-4, fig.1-5, fig.5-1 and fig.5-2, I strongly recommend you the reading of the following articles: New Open Graph tags for media publishers & Using Author Tags with Facebook Story Previews
These articles will help you do decide about your author/publisher strategy.

If you wish each author to be linked with his publications on the facebook stream you can add his own Facebook profile URL and Twitter @username in the Contact component. To perform this, just create a contact for each user for which you would like to override the general settings.
screenshot of the contact creation
Figure 4

  1. Create a new contact
  2. Give him a name and link it with a existing user (fig.4-1)
  3. Go the social tab (fig.4-2)

screenshot of the contact creation social tabFigure 5

  1. Provide Facebook (fig.5-1) and Twitter (fig.5-2) credentials.
    Note that Facebook must be an Url and Twitter a @username
  2. Save the new contact.

You absolutely don't need to use the contact component except once to do what I said previously. I just use its database table to store the user social profiles. The contact doesn't need to be published. You can even disable the component if you wish ;)
I made this choice to avoid having to create a user plugin which may conflict with other extensions which will also override the user management.
It may change in the future but that's the most reliable solution I found up to now.