WASHINGTON - Facebook CEO Mark Zuckerberg met one-on-one with a series of key lawmakers Monday. This could be the most important stop on Zuckerberg's apology tour. 

The Facebook founder swapped his trademark T-shirt for a suit Monday and waded through throngs of reporters. He ignored a question about what his message to lawmakers was.

His goal is to grease the skids before two days of hearings where he will be the sole witness, explaining to senators like Florida Democrat Bill Nelson how several outside companies managed to harvest data from unsuspecting Facebook users.

"If we don't do something now, none of us will have any privacy anymore," Nelson said.

Nelson said Zuckerberg was "forthright" but also "naive" for failing to act more quickly when political consulting firm Cambridge Analytica accessed personal information from up to 87 million Facebook users in 2015.

Nelson said Zuckerberg didn't explain why Facebook kept the incident secret.

Facebook on Monday began alerting affected individuals more than two years after the incursion. It also announced it was forming an "independent election research commission" to examine the effects of social media on democracy. 

Zuckerberg is expected to tell Congress that Facebook's mission of connecting people isn't good enough anymore, and that the company has to do more to make sure those connections are positive. 

The Associated Press contributed to this report.