Today it's easy to assume that social media platforms are a recent development, a phenomenon unique to the Internet age. But the exchange of media along social networks of friends and acquaintances is in fact much older than Facebook, Twitter or MySpace.

Consider the situation in the late Roman republic, in the 1st century BC. 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 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 one another 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. Letters were commonly copied, shared and quoted in other letters. Some missives were addressed to several people and were intended to be read aloud or posted in public. When Cicero or another politician made a noteworthy speech, he would distribute it by making copies available to his associates, who would read it and pass it on to others. Many more people might then read the speech than had heard it being delivered in person....