"As a result of a Facebook bug, entire domains were redirected," said Soltani. "That's an incredible amount of power to have."
Facebook presumably isn't going to be introducing similar bugs on a frequent basis. But the online ad industry deals with code trust problems every day.
Websites allow ads to be injected into their webpage code in the same way that many sites allow Facebook Login code to be injected. And in the ad business, the tenuous chain of connections across multiple domains and servers makes trust much more difficult. Cyber criminals routinely attempt to subvert ad network security, through hacking and fraudulent business practices, to distribute malicious code.
Soltani said that combined with other malicious activity such as DNS hijacking, reliance on third-party injected code can be even more of a problem.
There's also the issue of performance. Until December 2009, Google Analytics -- incorporated into many websites -- wasn't asynchronous, meaning that calls to Google's servers made webpages load more slowly. Other widget code inserted into webpages often has a similar effect.
In a blog post last October, Aarron Walter, director of user experience at email marketing service MailChimp, argued that social logins aren't worth it, a position others have taken before him. The social login buttons offered by Facebook and Twitter, he said, put security in someone else's hands and present the risk of brand damage if problems occur.
Moreover, the rationale for implementing social login buttons -- user convenience and ease of use -- might not be justified. Walter found that a 66% decrease in failed logins and a 42% decrease in password resets at Mail Chimp last year were the result of user interface design improvements and better programming, or "better error handling and copywriting" as he put it. Social logins contributed to only 3.4% of the drop in login problems.
Further complicating the question of whether to use a social login is the finding that many companies have implemented Facebook Login ineffectively. Research published by marketing consultancy Social Labs in September 2012 indicates that among the top 500 online retailers, only 30 have implemented a social login -- 77% of these being Facebook Login -- and a fair number of these have done so incorrectly. For example, 30% of the 30 online retailers using a social login require a website password in tandem with a visitor's social username and password, and 15 of these online retailers don't offer a social login during the checkout process, making for an inconsistent user experience. Finally, none of online retailers surveyed have implemented Facebook's login persistence, which automatically logs users in when they return to the website.