"I've known Mark since he was 20," said the former Washington Post chief who met Zuckerberg in 2005 and was on the board of Facebook for six years until 2015. "Mark is a good, decent, human being."
"The conversation has to stop there," Graham told CNBC's Becky Quick in a "Squawk Box" interview. "I'm prepared to listen to anybody who says Facebook shouldn't do some of the things they're doing. I'm not prepared to listen to anybody who tells me Mark is a terrible person."
Zuckerberg and COO Sheryl Sandberg came under heavy criticism for being notably silent for days after reports on March 17 that Cambridge Analytica had harvested the data from millions of users of the social network without their permission.
The data were used to target ads promoting Donald Trump's 2016 candidacy, according to The New York Times and The Observer. Cambridge Analytica has denied any wrongdoing.
Zuckerberg and Sandberg eventually went on an apology tour, and Zuckerberg testified about the matter on Capitol Hill.
Graham, a longtime supporter of Zuckerberg, urged critics to put themselves in Zuckerberg's shoes.
"Just think what it would have been like for you ... to try and run Facebook," he said. "Could you have done it? I could not. Not close. I don't think anybody else could have built and I know Mark will do."