Adsense arbitrage / Adsense Cpc Drop !

Hi everyone!
I'm new here and this is my first post … I'm currently running a website using adsense arbitrage (facebook traffic). When I first launch the compaign in facebook everything goes well in adsense but after 3 or 4 days the cpc drops by almost 75% (usually it starts dropping from friday) . And this is happening each time I start a compaign. I really don't know what is the problem knowing that I don't change anything in the website it just drop suddenly. Is it normal or should I…

Adsense arbitrage / Adsense Cpc Drop !