Does the Facebook app use HSTS for Android/iOS? No. It doesn’t need to. HSTS is specific for telling web browsers like Chrome, Firefox, etc. to interact with their web service via HTTPS only.
A mobile app will have HTTPS hard coded in the app itself making it the only option. I believe Apple requires this for any apps and for Android it looks like they are moving towards HTTPS for apps as well. Android HTTPS.
If you are developing an application yourself and want users to only use HTTPS then you would simply call your backend services using only HTTPS and have that coded right into your application whether android or iOS. Example: https://api.example.com/data
The reason why HSTS isn’t needed in the mobile app environment is because in the web browser a user can go to a base url like example.com over HTTP. The user has more control in a web browser environment on how they want to navigate to your website. Also indexing and back linking to your website, people can specify which protocol HTTP or HTTPS when linking to your website. So that’s why you need HSTS in a web browser environment to tell the browser, “only talk to me in HTTPS even if someone says they want HTTP” but in a mobile environment, as a developer you are the one that gets to specify HTTPS in the code in your app for interacting with the backend service. Users don’t need to manually navigate to a url because that is all coded in the app Facebook or you built. The users just have to open the app. So as a developer you get more say on how the app should interact with your web service. It’s your app, your client, you tell it what to do but you didn’t build Chrome or Firefox, it’s someone else’s client so you need to tell them via HSTS how it should interact with your web service.