Every tribe needs a blog, and a social network assigned to it. You can assign 1 blog, and 1 of each social network (meaning 1 Twitter, 1 Facebook and 1 LinkedIn account).
To assign a social network to a tribe, first click the "Settings" link under your Account drop down.
On the settings page, select the "Social Networks" tab.
On the Social Networks page, click the "Assign Tribes" button.
Next to each tribe, select which social network you would like to assign to that tribe.
As soon as you make your selection, the selection is saved. So once you are done, just click "close".