How could I avoid the facebook login part when my site is used as a facebook app

Hi,

I was wondering how I could avoid the login part when my website is used as a Facebook app.
When the website is used normally, they have to click on the button in the sidebar to login with Facebook. And that's working great.
On the other hand, when they are on Facebook, using the website as an app, I'd like to avoir forcing them to click that button because, they already are (indeed) logged in on Facebook.
Do you have a clue to get it works? Thanks a lot !

Regards,

Mathieu