The committee had asked for a top executive to testify — either Larry Page, Google’s co-founder and the chief executive of its parent company, Alphabet, or Sundar Pichai, the chief executive of Google. Google instead offered its chief lawyer, Kent Walker, who it argued was better versed in the issues. That didn’t fly with the committee, hence the spectacle of the empty chair.

Google has spent years building a lobbying operation in Washington, which rests on a foundation of seriousness and good will; while upstarts like Facebook reveled in their break-things disruptive style, Google positioned itself as the grown-up in the room. It also had some positive facts on its side: Google’s services were far less vulnerable to Russian misinformation during the 2016 election than Facebook or Twitter. And because it does not run a social network, Google could credibly argue that it was a less important vector for propaganda and social unrest. Mr. Pichai, who is no dummy, could have persuasively made that case to lawmakers.

But in declining to participate, Google left a sour mood in the Capitol. A parade of Democrats and Republicans in the Senate hearing noted the company’s absence. Many raised questions about Google’s recent actions — for instance, its decision to stop working with the military on artificial intelligence projects and its exploration of a censored search engine for the Chinese market — that the company had no way of defending.

In a statement, Google said of offering up Mr. Walker: “We had informed the Senate Intelligence Committee of this in late July and had understood that he would be an appropriate witness for this hearing.”

Jack Dorsey of Twitter overflowed with candor.

One worry among Twitter employees was that Mr. Dorsey would bow too far toward Republican lawmakers’ trumped-up charges of bias against conservatives. He did not do that. What he did instead was more useful. In several answers, Mr. Dorsey explained the many ways in which Twitter had failed its users.

My notes from the hearings are full of instances in which Mr. Dorsey admitted serious flaws. “I believe if you were to go to our rules today and sit down with a cup of coffee, you would not be able to understand them,” he said at one point.

He also said Twitter’s verification system — the way it determines which users get blue check marks next to their names, signaling that they are V.I.P.s on the service — was broken and needed a lot of work. And he said that the company’s system for reporting harassment asked too much of victims, and that the company took too long to take down disparaging content, like a doctored photo of Meghan McCain that made the rounds this weekend.