Rumors can happen online or offline and provides the same effect. Tell a group of 4 people in a school a lie can spread to the entire school population in an hour.
Your saying social media and the internet in general makes this effect worse. I agree it can speed up the messaging but it always starts from those same 4 people. And the same people turning on you because of an online post are the same group who will turn there backs on you for an offline rumor.
In both cases its bullying.
On the other hand the same internet and social media allows you to connect with other kids from outside of your local groups enabling a wider social net.