We use facebook so that you can start sending files to your friends with minimal friction -- gmail integration is included as well.
There are lots of people for which requiring Facebook is a complete non-starter. From the front page, it looks like FB is required.
For some people, deep Facebook integration makes things easy... but I suspect for your audience (early adopters, HN types), focusing on FB may be a bad plan.
I don't see mention of GMail integration on the site, and when I click "Get Started" or "My Files" it takes me to Facebook. So I assume then that Facebook is required, and gmail contact integration is in addition to Facebook contacts. Is that right?