You'll lose a lot of users as people REALLY do not trust social logins anymore due to sketchy security settings in Facebook.[1][2] Anecdotally I actually overheard an older gentlemen behind me at a coffee shop the other day lamenting about how he wanted to sign up for some service but they wanted his facebook login details and that "they posted all over my wall with ads last time and I'm not going through that again." Implementing your own user system isn't all that difficult as most technology stacks have a library to handle password storage.
[1] http://blog.mailchimp.com/social-login-buttons-arent-worth-i... [2] http://hyperallergic.com/54018/man-bartlett-why-i-deleted-my...