I am in the same boat. I have an S7 from one of the major carriers that came preinstalled with Facebook. You can disable it, but not uninstall. Also, there is an update to "Facebook Services" which I can't seem to find and disable. I absolutely will not get another phone from Samsung (or others) that comes with Facebook preinstalled.

>A preinstalled app is the reason to switch the platforms?

No. What they actually said :-

>Every damn Android phone I've bought lately has Facebook preinstalled, theres other things, but I'm done with it all.