Update (1445ET):

While the tech titans mostly stuck to their prepared remarks at the beginning of today's testimony, Facebook's Mark Zuckerberg veered off script - pointing out that Google is the largest online advertising platform in the world, while Amazon is growing the fastest.

"In many areas, we are behind our competitors," he said. "The most popular messaging service in the U.S. is iMessage. The fastest growing app is TikTok. The most popular app for video is YouTube. The fastest growing ads platform is Amazon. The largest ads platform is Google. And for every dollar spent on advertising in the U.S., less than ten cents is spent with us."

Translation: Facebook couldn't possibly commit antitrust violations with these advertising behemoths at the head of the pack.