It’s not as hard as what people think.

You just tell your friends that you are not reachable on Facebook anymore and how you can be reached.

You don’t have to "convince" anyone.

Your real fear is that people will stop reaching you if they don’t want to install another app, send you an SMS, send you an email or to call you on the phone.

Well, if your friends stops reaching you because you uninstalled an app, honestly it looks that they are more acquaintances than real friends. And it’s ok. But it’s also ok if they are just an entry in your contacts list.