Who invented social media? It wasn’t the founders of Facebook, or Twitter, or even MySpace or Friendster. Social media—the exchange of media within networks of friends and acquaintances—is much older than the Internet. A social-media environment requires two things: a certain level of literacy, and the ability to copy and deliver information cheaply and quickly. This combination first arose in the late Roman republic of the 1st century BC, more than 2,000 years ago.

At the time there were no printing presses and no paper. Instead, information circulated among the intermarried families of the Roman elite through the exchange of papyrus rolls. The ruling class was well-educated and literate. And in place of broadband, which makes copying and sending information cheap and quick today, the Romans had scribes and messengers, many of whom were slaves.

The correspondence of the Roman statesman Marcus Tullius Cicero, the best preserved collection of letters from the period, shows that he and his friends wrote to each other constantly, recounting the latest political machinations, passing on items of interest from others and providing commentary and opinion. “I sent you on March 24th a copy of Balbus’ letter to me and of Caesar’s letter to him,” Cicero wrote to a friend in one typical example. Caesar’s missive was, in other words, sent to Balbus and then copied to Cicero, who recopied it in turn....