how to login users if query is correct wordpress

Hi all

I am working on integrating facebook with wordpress, so far i have got this.

User comes to website, is directed to facebook registaration “on my website”

I store the returned data into the wordpress database.

The problem i have now is how do i automate the process off wordpress login.

I want wordpress to check if the users details that are stored in the db and then if they are automatically log them into the web site???

Stuck!!!! help pls