There's emerging evidence that emotional contagion via social media is a real phenomenon, and indeed much commercial exploitation of social media arguably depends on that. That has many positive aspects, such as meeting my appetite for heartwarming videos of cute animals, but it can also be leveraged for less wholesome purposes.

Definitions are absolutely fluid, not least because the science of measuring and mapping such contagion in virtual spaces is currently in a fairly rudimentary state. We're building our technological infrastructure out much faster than we are developing an understanding of how it impacts our future.