WASHINGTON, D.C. (WNAX) – Leaders from Facebook, Google and Twitter faced questions over how they handle content on their sites at a Senate Commerce Committee hearing this week.

South Dakota Senator John Thune says political bias from those sites is a real concern.

Thune says its difficult to find out how the various platforms allow content to be posted.

Thune asked Facebook chief Mark Zuckerberg about their role in content postings.

Thune has introduced two bills that he says would improve online transparency and accountability.