They don't duck surveillance from facebook as you and I know it, but from facebook as an employer, which is very different.
If a company provides your phone, then it has access to it, and if you don't want your company to have access to what you do on it, you use a different phone. If you want the convenience of a personal phone that you can check your work mail (or work facebook groups) on, then you grant access to that company.
It isn't any different from when a law firm or bank provides its employees phones. If you don't want that firm or bank to read what you write on it, then you need another phone.
A "burner phone" gets thrown away--"burned"--to prevent the someone from tracking you. You don't allow anyone to know you use it.
No one at facebook cares if you use a second phone. So some better terms would be "personal phone" or "non-corporate phone".