Facebook CEO Mark Zuckerberg met with Democratic Sen. Bill Nelson of Florida on Monday ahead of his congressional hearings on privacy concerns.

Nelson left the meeting believing Zuckerberg is sincere in fixing any privacy issues, but he's pessimistic about progress coming from Congress.

WASHINGTON — Mark Zuckerberg, the founder and CEO of Facebook, met with Democratic Sen. Bill Nelson of Florida on Monday ahead of his hearing before the Senate Judiciary and Commerce committees scheduled for Tuesday afternoon.

After the meeting, Nelson suggested that Zuckerberg's hearing could lack substance and that Congress was unlikely to pursue legislation to regulate tech companies' data collection significantly.

Zuckerberg, who is speaking with a handful of lawmakers before his congressional hearings over the next few days, spent more than an hour in a one-on-one, closed-door meeting with Nelson.

"My personal opinion of him was that he was forthright and honest to the degree that he could," Nelson told reporters afterward. "But I think there are going to be a lot of challenges for his company in the future."

Nelson said he explained to Zuckerberg that because of scandals like the misuse of Facebook user data by Cambridge Analytica, assurances of privacy from major tech companies alone would not be enough.

"You can't protect our privacy just on the basis of somebody telling you that they're going to protect," Nelson said. "So does that mean law? Yes. Does that mean regulation? Yes."

But Nelson stressed that he did not anticipate any new laws or regulations hitting the books soon. Of pushing such legislation through Congress, Nelson said, "Right now, I think it's going to be very difficult."

He added that his confidence in the security of Americans' data going into midterm elections this year was "to be determined."

Zuckerberg's testimony on Tuesday will be in a rare joint hearing, where dozens of senators will have a fixed amount of time to ask questions of the billionaire tech mogul. Nelson expressed frustration that it may be conducted too hastily.

"How in the world can you have 44 senators do a hearing that has a lot of substance when each senator has only four minutes?" Nelson said. "I was an objector to it being a combined hearing. It ought to have been a Commerce Committee hearing and Judiciary, where each committee could operate in its own way."

He added: "It doesn't seem to me that it's going to be a format that you're going to get a lot of substance, because the four minutes run out and you've got to move on."