Same issue here. I've had more than 15000 daily pageviews for years and has a Pagerank of 5. But since 2012, pageviews have been slowly dropping. In 2013 it's been a huge drop (-70%), week by week.
I have 5 websites, only 2 of them had more than 10000 daily pageviews, and are somehow similar (same topic). But the drop has been similar in all of them. All of them are either forums or have a lof of original content.
This is a big problem to my financial situation, since I've been living from advertising from years.