A little more than an hour into his testimony before a rare joint panel of the Senate’s Commerce and Judiciary committees, Facebook chief Mark Zuckerberg was hit with a question for which he did not have a ready answer.

The question was not very complex. It didn’t require a lot of details about the inner workings of the company. In fact, it was probably one of the most basic questions facing Facebook and other digital giants.

“Who is your biggest competitor?” Senator Lindsay Graham asked.

“Senator we have a lot of competitors,” Zuckerberg said.

“Who is your biggest?” Graham asked.

“Do you want just one? I’m not sure I can give one. Can I give a bunch?” Zuckerberg said.

After a nod from Graham, Zuckerberg began to layout a series of three categories of companies, the first including a list of familiar tech companies–Google, Apple, Amazon, and Microsoft–which he said “overlap” with Facebook. The Senator from North Carolina, however, cut him off.

“Do they provide the same service you provide?” Graham said. “If I buy a Ford and it doesn’t work well, I can buy a Chevy. If I’m upset with Facebook, what’s the equivalent product that I can go sign up for?”

Graham went on to explain that “car companies face a lot of competition. If they make a defective car, and it gets out into the world, people can stop buying that car. Is there an alternative to Facebook in the private sector?”

When Zuckerberg began to argue again that various other tech companies provide services that overlap with some of what Facebook does, Graham cut him off again.

“You don’t think you have a monopoly?” Graham asked.

“It certainly doesn’t feel like that to me,” Zuckerberg said.

That last answer elicited audible laughter in the hearing room.