The proportion of irrelevant, off-topic, inflammatory, nonsensical, spammy, screaming, content-free, trolling, or otherwise useless statements to actual useful, worthwhile communication.
From the informal sense of signal to noise ratio
, but inverted to emphasize the "noise".
"That discussion board used to be a great place to talk about politics, but now the noise-to-signal ratio is through the roof!"
"Typically, a demagogue actually benefits from nonsensical, contradictory and inflammatory statements and agendas. Ironically, the higher the noise-to-signal ratio, the more effective they become since any useful information would actually rein them in."
(adapted from Raul and JonG)