Question2answer FB Login option (maybe a plugin) is seemlessly working fine, except, it automatically creates user in website when a new user visits the website while logged in to Facebook in another tab of the browser (or tabs closed).
That means, even after these Facebook users not intended to register to the q2a website, they have been automatically logged in to the site and a username is created with 100points already. This will be a big problem, when all users with FB logged in just visit the site through a Google search. The database would be filled so fast.
For new sites, this seems a good option so that people will ask questions or answer questions without pressing login option. But, it looks like it will be a privacy issue as well.
See for yourself here. Just visit my site: askitfast.com when you are logged into Facebook in another tab. Your user ID will be created automatically.