Your site is beautiful, the kind of site I would share with my friends because I want to.
But a lot of the things I tried don't work properly. For example, I tried to heart a photo but clicking on the heart icon didn't do anything. So, I realised maybe I need to sign in first. I connected with my Twitter account, but then instead of taking me back to the photo I was looking at, I got a page asking me to share the site with my friends. Anyway I went back to the photo, but I still could not heart it.
Then I saw a few comments on a photo and wanted to add mine. But it said sign in to comment. But I thought I just signed in with Twitter. So I click on sign in anyway. And it turns out just connecting to Twitter does not sign me in. There's another step involved. Why is that?
I think you need to focus on the user experience and make it smoonth, simple and enjoyable. Then people will share it automatically.