Nata, I could be mistaken, but I haven't noticed you posting before , so welcome! You sound a very caring gran.
I don't mean to sound harsh and I understand where you are coming from, but in my opinion, you are overthinking this, times have changed dramatically from when our children were young.
I would tend to ignore it, there are worse things to worry about with teenagers. The more you go on about it, the more they will do it.
You obviously love your granddaughters, but take a step back and just enjoy their company, you don't want them to remember you as a nagging gran. They will soon tire of it. It's their parents role to deal with this if they think it is getting out of hand.
As a teacher, involved in IT, I found these things were a usually phase. Children discovered new technology and everyone was using it, They soon got bored and moved on to the next new App.
Try not to worry.