[steeleduncan]

The problem isn't that social login buttons harm your brand or look ugly, it is that by using social logins you are working to expand the social networks user base and not your own.

Online companies are largely valued by the size of their userbase and by working to build Fb or twitter's userbase rather than your own, you are sacrificing the value you add to your own company for the sake of the social network that a user signs in with.