The two senators got in an argument about the wisdom of spending U.S. tax dollars on national defense priorities. Rubio called Paul "a committed isolationist" and argued that the world is a stronger and better place when the U.S. has the strongest military power in the world.

Paul challenged his credentials as a conservative.

"Can you be a conservative and be liberal on military spending?...I want a strong national defense but I don't want us to be bankrupt," Paul replied.

Defense wasn't one of the top issues discussed during the debate, which was focused on the economy. Facebook users spent their time talking about taxes, immigration, the minimum wage, jobs and health care.