bitcoin banking is probably ponzi. simply because you are hardly earning a decent interest with the blocks halving, really man.
even with the mining, you won't gain a lot of interest in bank; again, halving.
Everything that promises you a certain interest can turn into a ponzi scheme at any time. Even if they had "good" intentions when they started. It's very simple, never let your coins be held by a third party. If that is your basic rule, then you will be just fine.