Android still does not support logging in with accounts created by signing in with Facebook, despite dozens of support threads and complaints... in case anyone at MS reads this, it sucks.
Single sign-on. It's really nice to NOT have to copy-paste usernames and passwords from password managers into native apps but use a simple "sign in with FB/Twitter" and it's done. 1 second with a decent internet connection vs 60+s for switching to Keepass, entering the wallet password, actually finding the password, copying the username, switching back to $app, pasting the username, switching back to Keepass, hope it didn't for some reason decide to randomly lock, copy the password, switch back to $app, hope it didn't forget the username in the meantime...
edit: it becomes a special annoyance when an app decides to lock you out after n days (n usually = 30). Great from a security POV but hell is that annoying if it happens to multiple apps at once on a day. Or when you have 20+ apps with unique logins when you reset your phone... as login credentials/tokens are (rightfully) not cloud synced. Android desperately needs a password-autocomplete API, secured by e.g. the fingerprint sensor found on new-ish Samsung devices.