It is not always possible to make an automatic connection between a new facebook account and an existing q2a account, because there are multiple things to take into account (are email addresses provided in both accounts, are they the same, are they verified, is this what the user wants?, etc).
This is the reason I chose to implement the automatic detection as an optional step when logging in with an external provider, even if the code could detect that the same person has another q2a account. After all, the end-users should have the last word on whether to use a duplicate account or link their accounts, so it should be their own choice.
It is also difficult to implement a generic way to do it otherwise, since different sites have different needs. Therefore every site administrator who want to enforce their users to not use duplicate accounts are free to use their own implementation of this plugin (code is open-source). If anybody comes with a feature which is generic enough so that everybody could benefit from it, I would be happy to include it in the plugin (just create a new ticket or pull request on github).
If you need more information on how the plugin works, do create an issue on the github issue tracker, and I try to provide all the details you need.