Hello,
Really glad to be here on STM, this is my first post
About me: I have won a national AdWords PPC contest in my country(centered around Google Search), after which I worked for a month in PPC(Search and Display mostly) at one of the best PPC agencies in my country, and now I am in charge of software engineering and ppc automatisation there. Given my PPC expertise, albeit none in mobile and limited in Facebook Ads, I am unsure regarding which traffic source would be most profitable for me to pursue.
From what I read on STM so far, the contenders are the following:
1. Facebook -> I read Zeno's guide which seemed really thorough and combined with my expertise so far I personally think it would be the best option to start with. Concerns: is it harder to earn a lot of money on Facebook via affiliate marketing(I'm talking only whitehat FB) than via media buying? Also, I read the mobile cookbook and I understood from it that mobile is much more future facing than Facebook and I wouldn't like to build expertise in an area which could soon be outdated or lose value so what are it's future prospects in term of AM compared to mobile would be my second concern.
2. Mobile -> Seems a lot more future facing, although I have no experience in it and from the threads I read I was left with the impression that profitability takes a while to reach. It is also unclear to me how it is compared to Facebook in terms of earning potential via AM, if somebody could help me here it would be great.
3. Pay Per Call -> Since I already have expertise in AdWords Search I think this would be the easiest one. I am unclear however regarding its earning potential compared to the other two and its future prospects.
I am really curious which is the best option for me, so I can get started as soon as possible. I know it's a lot to ask to compare those three from multiple points of view but it would be extremely helpful and I promise to help back whenever I can 
Thank you!