How can I monetize my app without losing my users trust?

I have an idea for a web app that will allow people to easily share links with friends / whoever. Basically the app allows people to register, create an address book with friends emails, cell phone numbers, and possible social integration. Once you've done that you can send friends links easily to their email or cell phone all from the same place. I'm most likely going to build a browser extension for sending links. I don't want to make any money from the app I just want it to pay for itself (hosting, twilio, etc.) I've looked into advertisements and I don't think it's the right fit because most people won't be spending a lot of time on the actual site as there's no real content. I thought about a paid subscription or a tiered plan based on usage and average costs. Are there any other ways to monetize a simple app?