I am under the impression that, unlike magnetic storage, once data has been deleted from a flash drive it is gone for good but I'm looking to confirm this.

This is actually relating to my smart phone, not my computer, but I figured it would be the same for any flash type memory. Basically, I have done a "Factory Reset" on the phone, which wipes the Flash ROM clean but I'm wondering is it really clean or is the next person that has my phone, if they are savvy enough going to be able to get all my passwords and what not?

And yes, I am wearing my tinfoil hat so the CIA satellites can't read my thoughts, so I'm covered there.