Guest Users on native mobile apps?

Hi all,

I am designing an app which I aim only to release as a native app. I plan on having a ‘connect to facebook’ as optional, but want to save/fetch user data for a user even if they haven’t signed in with facebook. I note on some mobile apps and games I use this is possible, and I’m wondering if this is handled at the device level? Otherwise, I will have to make signing in compulsory - but I want to see how this will work before going too far one way or the other. My preference is to have it as optional if I know that I can save and fetch data for the user as a returning ‘guest’?

Not sure if that makes sense to anyone, but feedback most appreciated.