Oh cool! So based on your comments, you've done an audit of all related code repositories and libraries to confirm?
I'm sure a Facebook engineer COULD find out if Facebook is doing this - but just working at Facebook is not sufficient enough to say for certain it does not happen, you need to actually do the required investigation.
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I don't personally think Facebook is listening in the way suggested here, key word would be _think_ though. And unless you're saying you've audited the code repositories, you're limited to your _opinion_ based on the context you have from working within the company.
You cannot say it doesn't happen with any level of certainty (as you've tried) just because you have access to the code repositories.
I used to work there a while (5 years) ago, and I don't think you understand how this company works.
There are internal groups where features are discussed and debated all the time, and even all hands Q&A sessions with Zuck (weekly IIRC) where sometimes hard questions are raised.
Someone amongst the many engineers with access to the repo will eventually end up spotting shady code if it exists. They might raise this with a colleague first, or in an internal group, and the matter might even bubble up to the Zuck Q&A if it's intentional and particularly controversial.
This level of internal openness is unusual at this company size, and since there are people joining & leaving all the time (including interns who AFAIK have the same level of access to the code) it would take just ONE disgruntled whistleblower to ignite a scandal that the press will be instantly all over like piranhas smelling blood, this being Facebook.
Therefore, doing evil covertly like you suggest just isn't worth the risk for FB, period. Not happening, won't happen. When you see headlines like "Is Facebook eavesdropping on phone conversations?", Occam's razor is always your best friend.
It doesn't require ALL repositories, just those that contribute to the mobile app. I don't know about Facebook, but where I work there are hundreds of employees who have access to the source code of our mobile app and a few tens of employees who know the deployment process well enough that they would know if other code were being added as part of the deploy process.
You work for a company with 1000s of employees. How are you exactly in a position to confirm this doesn't happen?