Given the market value of the company, then it should be able to comply with basic standards of say decency, honesty and perhaps ... transparency.
The other argument is that the major messaging platforms have collectively become an essential basic utility, and accordingly carry some responsibility.
I've been asking friends to get off WhatsApp forever, and everyone wants to get off it. But family members and clients are on it, so now everyone downloads multiple messaging apps, ones they want to use and the ones they're forced go use.
At this point even if WhatsApp explicitly says they'll sell your information to Satan, people will still use it because it's free.
Taking on so many users means you get responsibilities to society that smaller companies don't get.
Crowing 'private company' doesn't absolve you of providing the functions of the town square when you've bulldozed and replaced it and repeatedly stated that you are the town square.