Unlike infectious diseases and news, behavior change spreads faster through online networks that have many close connections instead of many distant ties. Redundancy is key, as people are more likely to engage in a behavior if they see many others doing it.
In other words, when starting a new, healthy behavior (say exercise), knowing ten random people who run may help motivate you. But if that group of ten people is a close-knit group of running buddies who all know each other, the chances of sticking to your new routine go up. Way up.