That's a very interesting topic, locally paid political astroturfers sloganeer so much at each other, that everyone else has been driven off the local news website comments section. Too toxic. People who want to comment on the news have to use FB or other social media. Another similar system might be spammers and flakes on CL, or spammers on whatever is left of usenet in 2014.
I'd theorize that the presence of spam shrinks the total pool while the ratio remains the same, but this is a very weak gut feeling.
I think an academic paper on the topic of spam suppressing interaction would have interesting financial implications, as the only thing keeping the lights on at the local newspaper (and some other places) is ads from pageviews, so spam may have a modelable financial impact on those operations. Depending on the result, this might either encourage new anti-spam startups, or wipe out the whole spam filtering industry (and probably, discussion forums as well)