I counter with the calm, but firm and frustrated assertion that just from a few postings those comments would have caused such a rash of inexplicable, evidence-less slasher murders inside still-secured dwellings that the news media would have gotten wind of them and mentioned them.
Further... unlike a chain letter, each posting of the message targets not a specific recipient but anyone who reads it. And it stays there until removed. If that was for real, you would have selfishly risked the deaths of endless people to save yourself. Even Sadako's cursed videotape from 'Ringu' didn't start playing itself to those doing something else near it, and each victim only had to make one copy to pass on.
Besides: the story was of a 'campfire' nature that combines several urban legends, which doesn't help credibility. And it came in a poorly proof-written text wall that gives you the impression whoever wrote it wasn't depending on it's persuasiveness to save their life.