Meta "acquired Instagram and WhatsApp to improve and grow them alongside Facebook," the company's attorney Mark Hansen argued.
A win by the FTC could force CEO Mark Zuckerberg break up the company.
That could include spinning off photo sharing app Instagram and messaging service WhatsApp.
The FTC says the company overpaid when it acquired Instagram for $1 billion in 2012. Two years later, it purchased the WhatsApp for $19 billion.
"The [FTC's] argument is the acquisition of Instagram was a way of neutralising this rising competitive threat to Facebook," says Rebecca Haw Allensworth, a professor of antitrust law at Vanderbilt Law School.
Ms Allensworth says Mr Zuckerberg's own words, including those from his emails, may offer the most convincing evidence at trial.
"He said it's better to buy than to compete. It's hard to get more literal than that," Ms Allensworth says.
On Monday, Matheson referenced a 2012 memo from Zuckerberg in which he discusses the importance of "neutralising" Instagram. Matheson called the message "a smoking gun."
Meta, on the other hand, said the purchases made the consumer experience better.
"Acquisitions to improve and grow" have never been found unlawful, Hansen said Monday, "and they should not be found unlawful here."
The company maintains that it faces competition from a slew of other apps, including TikTok, X, YouTube, and iMessage.
Mr Zuckerberg and the company's former chief operating officer Sheryl Sandberg are both expected to testify at the trial, which could run for several weeks.