The test plans define how functionality and elements of the site will be validated during development, in beta, and post-launch of the site. This helps to identify key problems before they cascade throughout the system. Typically, project elements are tested individually as developed and then in aggregate during beta.
User testing is conducted with both pointed questions and scenario testing.
So you know the impact of thousands of concurrent users before your users complain that your server is too slow. Write a test plan that includes realistic user workflows and the range of server loads to be tested.