I realize both are competing for ad dollars and can be called competitors for that fact [1], but my own use of them really doesn't overlap. I use FB for interacting with friends and family, I use Google for interacting with the rest of the web. FB's search is not optimal and Google+ is well... it's not something my parents or my kids are going to use. There's not much overlap in my video viewing habits either. I don't sit down to watch a video on FB. How would I even find something to watch on FB? YouTube search is incredibly impressive and satisfying. I'm sure FB and Google would like to take more of each other's markets, but I don't see how they would do that. I may be projecting, but I think most people are satisfied with two great services and don't feel the need for one that does it all.
The impact would be lower CPCs for mobile ads. People moving their ad spend over to Facebook from Google. One of the interesting myths people used to believe in was that adding a new advertising platform would add more ad dollars to the digital economy. However, the pool of ad dollars is finite, the people who spend them have fixed budgets so when they shift them to new inventory they take them away from somewhere else. And the way that is reflected in Google's number is their auction system for ads gets lower bids as fewer people are competing for various spots.
Thanks, I figured I was missing something since I don't know much about the ad business other than what I read about in regards to the impact of ad blocking and efforts such as Brendan Eich's company to mitigate the problem.
[1] http://www.inc.com/peter-roesler/google-vs-facebook-3-things...