One evening in March, library assistant Graham Mallaghan was leaving work at the University of Kent in Canterbury when he noticed a group of people hanging about outside the exit. Some of the group started shouting abuse. “Wait till he comes out, we’ll kick his f****** head in,” one of them yelled.

For Mallaghan this incident was part of a confusing pattern, in which he increasingly found himself being intimidated and threatened with no apparent explanation. No explanation, that is, until an acquaintance told him to look on Facebook, now the most talked about of the online social networking sites.