The demonstrators gathered in front of Zuckerberg cardboard cutouts chanting, "the internet is getting dark and we owe it all to Mark."
He's apologized many times already, to users and the public, but it is the first time in his career that he has gone before Congress.
Zuckerberg will testify before a joint hearing of the Senate Judiciary and Commerce Committees on Tuesday and before a House panel on Wednesday.
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