Facebook data collection ethical issues

Question

If I have a Facebook app, and my users agree to allow my app to access their information, photos, friends, etc, is it ethical to grab their information when they log in, and then saving it in memory so that the next time he goes to my app, it can load faster?

If so, what about when the user logged off? Is the right thing to do to is to delete all the cached information and photos that the user provided?

Has Facebook got any way to detect that we're doing this (saving their information, etc)?

EIDT: Just to be clear, Facebook's term and agreement is not very clear on this matter (agreeing to access information is not always equal to agreeing to have the information stored). As in where I'll be storing the data, it will be just in the user's disk, not my own server. So I can't guarantee that the data is being encrypted securely (If someone steal the phone, that someone will probably be able to get the data)

And yes, my intention is to give my users a better app experience, not anything else.

EDIT2: I'm torn, one answer with very high votes says it's ok because I'm providing a better user experience, but others says I'm breaching privacy. Can anyone provide links to the documentations? Or can more people vote? I'm really glad for the responses!

Answer 1

You're not being malicious. Providing the user a faster experience is beneficial to both the user and to you.

With that said, if the data is not stored on your server in a secure manner and you're being reckless or negligent with the security of that data, then that may raise some ethical questions.

Answer 2

I'd say that it would probably be best to have it off by default, but possibly prompt users to opt-in for faster load times. I think a lot of people would have a problem with you storing their personal data on your server for an arbitrary amount of time past when they sign out of the app.