Google is making all kinds of changes right now (as always) with Hummingbird, Encrypted Search and Penguin 2.1 so those would be good places to start searching (if you aren't familiar already).
I didn't look very closely but I notice you solicit advertisers, do you add the rel="nofollow" attribute to what could be construed as 'paid links?'
Google (via Penguin) has been really cracking down on links that violate their webmaster guidelines, and sites that sell and/or buy paid links are both liable to be punished. Review who links to you and who you link to, especially links that pass PageRank.
Here's a YouTube video (one of many) where Matt Cutts discusses Google's thinking on paid links and advertisements.
No idea if that is the case but a drop that big looks like a penalty of some sort.