Facebook no longer allows posting to a user's wall when they login to your site. That used to be something they allowed, and JFBConnect supported. However, they changed their policy on that about 2 years ago and no longer allow us to do so.
When it was allowed, the automatic posts to users walls were widely marked as spam, which lowered the visibility of all posts from your app.
I hope that helps explain. If you have more information on what you were trying to post on login, we may be able to provide you with some alternative ideas.
Thanks,
Alex