On Capital Tonight: Facebook CEO Mark Zuckerberg testifies before Congress on user data being used in elections. We have a report from our DC bureau reporter Margaret Chadbourn. Our Insiders Chad Adams and Jessica Holmes look at a potential trade war as a campaign issue in the midterms, and the other big issues of the day. We look at the state of the South, as a new report shows how the region is faring in education, economic development, and more.