I was hit pretty hard at lasr.net, a niche travel site focusing on attractions and events. While the site has been up for over a decade we are currently down 50% from where we normally are. We had the same issue of having placeholder pages where users could submit their own attractions, events, etc. While normally operating around 400k pages indexed, I did notice that we ballooned to over 1 million indexed weeks prior to the farmer update.
On the downside for us, the majority of our pages that ranked very well were original content that had been present on the web for a minimum of 5 years. 
I am aggressive on the site with adsense / advertising as it provides the revenue for site production and I could see those placeholder pages or pages with low content being targeted (not unfairly so) as their ratio was a bit skewed.
Currently we are focusing on reevaluating both on page and off page SEO with a primary push for unique content generation. In the last month we have seen slow, but steady, growth in search traffic.
