Since this is the 3rd thread you open about the same issue (2 here and 1 on askive.cmsbox.jp, all still "open" without a feedback from you) I was tempted to give to you a "-1" 
As I told to you, if you search here a bit, it's full of solutions to common issues.
Or, if you want to have more concrete help, you should post here your issue "and" your setup details (your facebook app and q2a detailed configurations).
However, if you want only a "general" info about facebook login setup, here it is (you can find it also in other posts of this site; it's the procedure that I've applied and worked for me on Ubuntu) :
On the Facebook app dashboard :
- in the "App Domains" field put : yourdomain.com
- in the "Website with Facebook login" section, in the "Site URL" field put (for example) :
This value must be equal to the value in the "Preferred site URL" field on q2a (admin panel settings). In this case the "Preferred site URL" of your site should be also : http://www.yourdomain.com
In the "Display Name", "Namespace", "Contact email", put the values you want.
Disable Sandbox mode.
- you do not need to fill the other sections ("App on Facebook" and "Mobile Web")
Take the Facebook "App-id" and "App-Secret" values and put them in the facebook plugin Admin panel options.
Save changes.
You have to have your curl correctly installed with your php.
(Have you configured it with the steps I've sent to you ?)
