SAN FRANCISCO — Two pages of notes sitting in a binder in front of Mark Zuckerberg during his congressional testimony this week hinted at a message the Facebook chief executive rarely got a chance to deliver: We’re not the only ones.

Mr. Zuckerberg was prepared to say that his company accounts for just a slice of the $650 billion advertising market and that it has plenty of competitors. Google, for example, has an online advertising business more than twice the size of Facebook’s. And Google also collects vast amounts of information about the people who use its online services.

But as Facebook has taken it on the chin over the way it has handled the personal information of its users, the leaders of other tech companies have demonstrated that even in publicity-hungry Silicon Valley, it is entirely possible for billionaire executives and their sprawling empires to keep a low profile.

What set Mr. Zuckerberg apart? The personal information of up to 87 million Facebook users ended up in the hands of the voter-profiling firm Cambridge Analytica, which worked with the Trump campaign in the 2016 presidential election. Google and the biggest tech companies — as far as anyone knows — have never made such a giant mistake.