User ratings, if done correctly, will make the site "sticky", so if they build their user base fast enough, they could have a first mover advantage. Even so, social networks supposedly have a first mover advantage and even those die off as improved user experiences are presented. It sounds like several of these dog sites have launched in the past few months, and the winner will be the one that makes the best user experience while building volume of users with polished profiles and ratings that seem real and credible.