Just for fun, I tried these domain names via the url get.app:
apple.app
facebook.app
instagram.app
twitter.app
ycombinator.app *
snapchat.app *
producthunt.app *
whatsapp.app
amazon.app
microsoft.app
google.app
hotmail.app *
dropbox.app
intercom.app *
pivotal.app *
tesla.app
dell.app
ibm.app *
* AVAILABLE