Some things just don't meet users' mental models. One example - when a user clicks on an image, a popup appears with that image enlarged and the way of closing it is a "Return" button at the bottom right. There is something else on the top right corner, where normally an icon for closing is. The "Return" button looks like a button for submitting something, which is misleading and requires uses to think - they hate that. Also, the login form shows no error messages when it should. The design is good, but much more work is needed to make it user friendly. A good start would be seeing how other websites deal with similar things.
Also, there is no way for users to know that it is a social network. If accidentally came across this website, I would assume that it is some sort of blog.