Files are stored only on your computers, there is no cloud part. Sync uses p2p to create a cloud from your computers. You will need to care about your setup, to have 24x7 availability of the files.
Turn things around. Instead of promoting your main product, create and promote related-to-that-area product that refers to your main product.
As an example, you have a hosting service. Open a site with a lot of web based tools like whoise, traceroute, so you will target right segment and use this tools site to promote your hosting.
Suggestions: Send not only card, but also envelope to put this card in and preferably address printed on envelope.
Just btw, you could think about sms or e-mail service. So instead of getting card, I will get e-mail or sms, so I could forward it to right person with slight modification in wording.
there are always two sides of the story. You spent a lot of time on that, this is your idea and your fifth kid. So you are scared that it will be stolen away from you.
However one you will pass this fear, and slowly show it to the world, you will face another kind of problem - how to promote your solution and do a PR noise. So you will spend enormous time promoting, publishing talking about the product.
Sure, if the site is compromised than nothing you could know. But this is kind of unlikely in case of major players like banks.
Zero knowledge web app - is nice theoretical approach but I don't see a way it will be adopted anytime soon.
As to mine claims, I will post application when it will be ready, and we could discuss attacks against of it. It will be quite different from clipperz etc.