To acknowledge a statement by not replying.
The assumption is that all involved heard/read what was said (as would usually be the case on small Usenet newsgroups), all agreed upon it enough not to object, and no one had any objections, so the statement is presumed to have been heard and acknowledged.
Guy: "Revenge of the Sith was the worst movie ever."
Group: "..." <-- usenet nod
Guy: "No one liked it? Good."