Has this been answered?
Anyway because you can't delete messages, people use that to self-censor after the fact.
It's used when you want to say, "I'm ignoring you, and I want you to know it." Similar to typing, "This Page Intentionally Left Blank." It's not really blank anymore, but it gets the message across that there's nothing to see there. If the page were truly blank, you would wonder if that was intentional or an accident.
Simply ignoring certain members only causes them to become more unhinged in an attempt to illicit a response. The hope was that a "dot" would convey the appropriate message without engaging the member.
It's hard to say whether or not the tactic has had the desired effect. It does, however, appear to act as a placebo for the person posting the "dot", so they aren't compelled to issue an actual reply. So, I think there may be some benefit to it.
Having said that, the double-dot is nothing more than piling on. I think simply hitting the "Like" button on the "dot" post would suffice.
All of this is my opinion. Whatever....