The positive connotation of virus in the marketing arena did NOT begin in social media. This Fast Company article describes the birth of "viral marketing" with Hotmail. (Like many stories it may not be entirely true, but it was written in 1999, pretty close to the actual events.)
However, I think the social media platforms (especially Facebook) have invested in antivirus, trying to control their revenue streams. Viral marketing was always about finding a trick to get "free" awareness or sign-ups. It was exciting to think by being very clever, we could avoid the system. The system however, abides! Long live guerrilla marketing, probably without viruses.
Medium: The viral publishing game is over and we all lost, 2016-Feb-23 by Gabriel Stein
Like every other zero-sum game, the newsfeed for publishers is now naturally minimizing and consolidating, because the advantages you gain by playing become baked-in over time. The more games you play, the more data you have about how to test and optimize for the next battles, the quicker you can react to future changes in the algorithm, the more games you win, and so on.
It’s not impossible for insurgents to compete in this environment, but it’s a lot harder and requires much more investment to gain a foothold. That’s why you’re seeing all but the biggest publishers top out at around 30 million uniques a month these days. There’s just too much competition and not enough newsfeed to do more than that unless you’re publishing a thousand times a day. Put another way, 30 million a month is the supply ceiling Facebook has imposed on the market.