Would enjoy having a bit of a dissection of what you were doing back in the 6 figure month days you mentioned you had, what campaigns you were running, what went wrong with it, etc.. (I figure it's been long enough now for outing not to be a problem?) I find honest thorough dissections of people's mistakes far more useful than just tips / advice without backbone.
(This post isn't meant to be a dig or sarcastic by the way, just to re-inforce, don't want to come across as attacking you or anything. I'm genuinely curious and think people can learn a lot from that kind of stuff.)
This will be have to be a rolling answer too like above.. let me know what more detail you want after I answer.
I was running all diet rebill on Facebook. No other ad network, no other niche, no nothing. All in on diet on Facebook, but I was both domestic and international. I also did end up branching into dating and biz op and a diabetic supplement, but those were not my BIG FIGURE days, but they were not small by any means either.
What went wrong? Thinking it would always work like before. I never counted on ad networks running these ads themselves or affiliate networks either, let alone the countless copy cats. Around the middle of 2010 ( I started at mid 2008 ) I felt the squeeze from all this and branched into Adwords, Prem PPC ( like adsonar ) and media buys and PPV with other niches. Those were not making me huge days though but did branch me out.
In the end, I realized that capping out offers and hosting going down and the random bans on Adwords, auto bidding on PPV, and carrying a laptop around with me 24/7 to tweak campaigns was not what I wanted anymore, even if I was making 10k a day or more. I bought a house, 2 new cars, went on vacations and set up my retirement fund and decided to just get a normal job where my wife and kids didnt have to live with a zombie anymore.
I did this all by myself, which was another mistake. No 1 man can do everything all the time. I tried, I lasted a long time, but I paid a heavy price for it. I went back to full time gigs doing what I did best, which was marketing and development. I hated "running" a business all by myself.
Ask me more detailed questions, I know this was a general overview. Keep them coming bro. Nothing off limits.