I too was curious of this balance weighting. FB slurps in all of the data that users voluntarily post. Google just learns things through inference about users whereas FB is getting data posted directly by the user. Seems to me that FB is able to be way more invasive.
> FB slurps in all of the data that users voluntarily post.
That seems likely to be a grand understatement. FB has the opportunity to collect a great deal of data about their users beyond what they explicitly post -- for example, data about when and how they use Facebook mobile apps, how they interact with the Facebook web site, and what external web sites they visit which contain Facebook Like widgets.
On the other hand, FB is inherently social. I assume everything I give to FB has a chance of being public one day. I have some private conversations, but in the back of my head is that time the UI was deceiving and made seemingly direct messages public. FB is for sharing things. Google runs my phone, my work and personal email, my calendar, and more. I think they have a better attitude toward it, hence my willingness to trust them so far, but from a standpoint of ability to be invasive, Google blows everyone else out of the water on my devices.
I can see your concern about messages via email, but I know for me personally, email is just not a thing anymore. Forgetting plain SPAM, corporations/marketing/etc have ruined email into this signal that has such a low S/N ratio that it's just not useful. What percentage of internet users actually use email for communication anymore? Sure, some, but it's not my largest attack vector (I consider Google/FB as attacking me).
Anything serious goes trough emails and this is the data I’d be most worried about leaking - anything from security related stuff like login/id confirmation to receipts, confirmations, sensitive data, professional communication.
Waaay more valuable than FB scraping my phonebook and photos
This may be true for personal communication but any sort of business deal is going to be happening over email. Mortgages, selling your company, large sales... All of the contracts are going to end up in your inbox.
