Do you have permission to be taking these snapshots? The Newseum does something similar for print content and has agreements with all of the organizations so that they don't just Cease & Desist them out of existence.
Doesn't Fair Use cover "commentary, criticism, news reporting, research, teaching, library archiving and scholarship."? This site seems non-profit and educational in nature, so I would imagine it would pass the balancing test if push came to shove.