Where to put OAuth logic?

I am using the Zend Framework in a project and I am creating a controller for authentication only. In this project, we assume that the user is logged in through the account of other sites, such as facebook, twitter, myspace, etc. For this we will use OAuth. But I doubt where I should put the logic for each authentication on the OAuth website? For example, only facebook took 50 lines of code in my controller, and thus my controllers will not be thin ...

I wonder where I have to put this. Create a Zend_Auth adapter for each of the sites, create a service just for this, which is best?

And sorry for poor English. :)