Sheryl Sandberg, the former chief operating officer of Meta, said in a landmark antitrust trial on Thursday that the social media giant faced strong competition and that it nurtured and grew Instagram after buying the app, countering accusations that the company illegally stifled rivals.

In her second day on the stand, Ms. Sandberg, who left Meta in 2022, said the company feared the rise of TikTok around 2020 and how the video app’s popularity could eat into Meta’s advertising revenues. In a document presented to the board of directors in 2020, she wrote that TikTok’s ascent could take $3 billion to $6 billion from Meta’s revenue as its users spent less time on its apps, which include Facebook, WhatsApp and Instagram.

“It’s a big deal,” Ms. Sandberg said in her testimony, referring to Meta executives’ concern for their business at the time. “Wall Street doesn’t particularly like misses of any size, but especially in the billions.”

The U.S. government has accused Meta, formerly known as Facebook, of illegally cementing a social media monopoly by buying young rivals such as Instagram for $1 billion in 2012 and WhatsApp for $19 billion in 2014. Meta employed a “buy-or-bury strategy” to consolidate its power, the government has argued. The government plans to seek a breakup of the company if it wins.

The case — Federal Trade Commission v. Meta Platforms — has so far hinged on a trove of internal documents in which executives discussed concerns about competition from start-ups. Mark Zuckerberg, Meta’s chief executive, spent 10 hours on the stand as the first and marquee witness in the trial, which started on Monday. Judge James E. Boasberg, who will decide the case, is presiding over the trial in the U.S. District Court for the District of Columbia.

The case may be challenging for the government to win because the Instagram and WhatsApp deals were approved by regulators more than a decade ago. It is also hard to prove the hypotheticals of what might have happened had Meta not bought Instagram and WhatsApp and whether or not they would have been as successful.