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The empty chair for Google at a hearing before Congress came with a series of unanswered questions.

On Wednesday, Facebook COO Sheryl Sandberg and Twitter CEO Jack Dorsey testified before the Senate Select Committee on Intelligence about social media companies and foreign influence in the US. Google, however, declined to send its CEO.

The committee had invited Larry Page, CEO of Google parent Alphabet, and Google CEO Sundar Pichai. But the search giant offered Kent Walker, Google's senior vice president of global affairs. That wasn't enough for Sen. Richard Burr, a Republican from North Carolina and chair of the committee. Burr rejected the offer, explicitly requesting someone at the highest executive level.

Google's absence means a major puzzle piece was missing from the hearing. The company has dealt with its fair share of election security issues, including a recent campaign by the Iranian government that included the creation of channels on Google-owned YouTube to spread propaganda.

We may view Facebook and Twitter as the two main social networks, but we shouldn't overlook Google's influence. It holds sway over a massive number of people: the users of more than 2 billion Android devices, 1.8 billion YouTube watchers, and more than 1 billion active Gmail users. And though Wednesday's discussion focused on foreign influence on social networks, political interference has also occurred on YouTube and in Gmail.

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Which is why the absent Google was a target of scorn throughout the hearing.

"I'm deeply disappointed that Google, one of the most influential digital platforms in the world, chose not to send its own top corporate leadership to engage this committee," Sen. Mark Warner, a Democrat from Virginia and vice chairman of the committee, said in his opening statement.

There are plenty of issues the senators would've liked to ask Google. What protections has it put in place to prevent future hacks of Gmail? How does it deal with conspiracy theories and hoaxes boosted in its search results? Why do disinformation and propaganda campaigns continue to pop up on YouTube?

When Russian hackers stole documents from the Democratic National Committee and John Podesta, Hillary Clinton's campaign chairman, the attack was carried out through a phishing scheme on Gmail. The hackers created a sophisticated page designed to look like a Gmail login page, where the campaign chairman unsuspectingly typed in his password, giving the hackers access to his account.

Google has added more features to detect phishing emails. But it wasn't on Capitol Hill to testify about them publicly.

Senators also wanted to question Google about harmful hoaxes that continue to pop up on YouTube. In February, a video claiming that David Hogg, a survivor of the mass shooting at Marjory Stoneman Douglas High School in Parkland, Florida, was a crisis actor became the top-trending video on YouTube. The company said it was a mistake.

That mistake has happened more than once. Conspiracy videos again popped up on YouTube following a shooting in April.

Foreign propaganda from Russia and Iran spreading on Google's platform is also a concern for lawmakers. In July, Google rolled out stricter policies for political ads. Yet watchdog group Campaign for Accountability found it could still buy ads with a Google AdWords account it set up using the name and details of a notorious Russian propaganda group -- ads it paid for with rubles.

Lawmakers were also expected to confront Google about issues with alleged political censorship, after President Donald Trump accused the company of rigging search results.

The issue came up only once during the Senate hearing, when Sen. Angus King, an independent from Maine, discussed it with Sandberg and Dorsey.

Though the public is missing out on all these pressing questions for Google, lawmakers are at least getting answers in private from the company. A Google spokesperson said Walker was in Washington, DC, and briefing members of Congress privately about its work.

Those private sessions are a disservice to the public, says Daniel Stevens, the executive director for the Campaign for Accountability.

He noted that Facebook and Twitter were transparent because they sent high-level executives to testify publicly. But, he says, it's concerning that Google wasn't willing to do the same.

"If Google just privately briefs senators, how can the public have confidence that they're doing enough?" he asked.

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