I don't think they will be hurt by the fraction that do leave for this reason.
Assuming someone already has knowledge of Facebook's past activities, then they are likely already gone. If they are still using the service, it's unlikely this will make a difference.
Cheap and convenient is just too much of a draw for some folks. Especially when it involves "staying connected" with friends and family (irrational behavior driven by emotion).
https://keybase.io/blog/chat-apps-softer-than-tofu
(I have no affiliation with Keybase, I just appreciate their very thorough and public analysis.)
I used the wayback machine to read the older version of this article, it took specific aim at WhatsApp and Sigal, indicating that (in so many words) it disapproved of the apps providing a notice of Safety Numbers changing instead of the app flipping its wig and making the user take affirmative action to continue communications, otherwise it wasn't true TOFU.
I have about 30+ contacts on Signal, and I almost __never__ get Safety number changes. I certainly don't think making the user click through yet-another-dialog-they-wont-read will be a big security improvement.
I suppose I disagree with the 'not true TOFU' argument.
But since this idea has been brought up by a number of ad companies (Google, most notably), I can answer in the more general sense.
I would not support ad targeting done that way. It is a bit less objectionable, but it doesn't really address my objections to the entire practice.
How about we techies stop kissing ass to the advertising execs and stand up to them for once??
They want more data, more relevant ads, more more more more more. Their ask is impossible without massive invasions of privacy. Fuck them, I say
Return to scattershot ads, they can understand me as an audience without having to mine every ounce of my life for it. That approach will produce more relevant ads for me anyway
I am not sure if I'd be willing to make the argument we should ban (as in make illegal) collection of user data, but acting like such data is required to run ads is ridiculous.