Yesterday my wife left her painting supplies at Angelia's painting class so she needed a easy way to let Angela know this. She didn't have Angelia's number so what should she do? -- turn to social media of course. Luckily Angelia uses Hangouts even though she has a group page on Facebook. If Angelia had only been using Facebook this is what could have happened.
Judy would have had to install the Facebook message app in order to text Angelia. The text may or may not have gone through, but let us look at the consequences of installing the Facebook messaging app. During the install process the app ask for permission to see all the info in your contacts app. Most people just click OK OK OK because they want to get the message to Angelia; but if you do that, Facebook now knows all the people you know, at least those you had contact info for. Angelia might be ok with Judy messaging her, but are all of Judy's other contacts in her contact app willing to let Facebook have that information? Everybody knows that Facebook keeps "shadow profiles" of the people you know who are not on Facebook and this is how they fulfill that database.
You might ask why does Facebook allow this? They would respond by saying "You are the person who clicked OK OK OK"
You could go outside Google's Play store by getting the app file, but most of those people get me to help them like they do with Tubemate (which allows you to download videos so you can watch videos when you don't have internet-- this app you can't get from Google play store).
So why don't people care about their's and their friend's contacts lists privacy? Probably, people don't know what Facebook is up to or they don't mind ratting on their friends who want to remain private.
Can this be true? Look at what Mark Zuckerburg told his investors." 1. Facebook wants to be the middle-man in all personal communication. 2. Facebook wants to make all personal communication public over time." This is why your Facebook privacy settings change over time. Look at the bottom right text on the above image. "Facebook may automatically add additional capabilities to each group" -- or to put this in English "Facebook can change your privacy settings whenever they want." This is a lot different then Google's Mantra, "Do no harm." For the best social experience join me on Mastodon.