In September 2006, Mark Zuckerberg, the chief executive of Facebook, described what made his social network special.

“Facebook is about real connections to actual friends,” he wrote in a company post.

Two decades later, that description is at the center of a landmark antitrust trial against Mr. Zuckerberg’s social networking empire, now called Meta, and whether it illegally stifled competition. In essence, the trial has raised the question of whether social networking is simply about connections to friends and family, or whether it is something more.

The Federal Trade Commission, which is prosecuting the case, has tried to narrowly define social networking as a service that links friends and family. Under that definition, Meta would really compete only with Snap, the maker of Snapchat, which it dwarfs in size and users. But Meta has argued that all social media companies count as rivals, especially TikTok and YouTube, which would mean that competition was more abundant.

“The friend part has gone down quite a bit,” Mr. Zuckerberg said in testimony at the trial last month, downplaying his words from 2006.

The opposing definitions of social media in the case — Federal Trade Commission v. Meta Platforms — illustrate how much social networking has evolved over more than a decade and how slippery it has become to pin down. Meta has expanded far beyond Facebook’s roots as a bulletin board for college students, and scores of newer companies have developed similar products, emulating popular features such as the “like” button and news feed.

In the first four weeks of the trial, a parade of social media executives from companies including Reddit, Pinterest and LinkedIn have done little to help clarify a social networking definition. They acknowledged that they all competed for the same users, but in many cases offered very different products.