
The appearance marks the first time that Zuckerberg has testified before Congress. Zuckerberg, 33, swapped his usual gray t-shirt and jeans attire for a dark blue suit and light blue tie.
He appeared somber as he walked in to testify, and spoke before a packed room, with 44 senators in attendance.
"It's extraordinary to hold a joint committee hearing. It's even more extraordinary to have a single CEO testify before nearly half the United States Senate," said Sen. John Thune, chairman of the Commerce Committee.
"Then again, Facebook is extraordinary."