Why are you so concerned with who your kids are talking to? You have to teach them trust else they’ll continue on passing on this mistrust you clearly take as normal. How did the kids exist prior to Facebook messenger?
Interestingly I’m married to the product of this sheltering. When anything bad happens at all, she panics. Like panic attack panics. Things happen when they happen, and you’re attempting to control that instead of teaching.
There will never be a proper age to ween them into the real world.