Why does Quiet only blocks websites in Safari?
Due to technical limitations set by Apple it is for app developers not possible to block websites system wide with a Safari Extension. It is possible for a VPN app to block websites system wide but in that case you give all your web traffic to that service. I do not want to know anything about your web traffic. With a Safari Extension like Quiet the developer doesn’t see any web traffic. That’s why blocking social media is limited to Safari only.
How can I block social media apps?
In iOS 12 there is a function called Screen Time which shows how much time you spend in certain apps. It also allows you to limit your time or block apps all together.
The Verge has a nice article on how you can limit/block apps in Screen Time. https://www.theverge.com/2018/9/17/17870126/ios-12-screen-time-app-limits-downtime-features-how-to-use