Pew Research asked respondents about six different forms of online harassment. Those who witnessed harassment said they had seen at least one of the following occur to others online:
60% of internet users said they had witnessed someone being called offensive names
53% had seen efforts to purposefully embarrass someone
25% had seen someone being physically threatened
24% witnessed someone being harassed for a sustained period of time
19% said they witnessed someone being sexually harassed
18% said they had seen someone be stalked
The full report is here and is sadly not surprising.