You can now link both your Google Account (including Gmail) and Yahoo ID with your Facebook profile.

The advantage of linking you other online accounts with Facebook is that if you are signed-in into any of the above services, you’ll automatically be logged into Facebook as well. So for example, if you working in Google Docs or writing scraps in Orkut and then decide to open Facebook in another tab, you won’t see the Facebook sign-in screen anymore.
To set up this association between Facebook and Yahoo/Google Accounts, just go to Facebook -> Accounts Settings and click "Add a new linked account".
Facebook lets you link only one account per web service so choose the ID that you use most frequently on the web.

Related: Managing Multiple Google Accounts
Find this article at: http://www.labnol.org/internet/facebook-autologin/8916/
Reader Comments
So Google will work with Facebook for this, but Facebook will still not work with Google Friend Connect? Lame.
Written by Tenkely on 05.31.09
This is such a nice feature but has a downside to it. In case a user’s google account get’s hacked, the hacker gets direct access to the users facebook account.
Written by Debjit Saha on 05.31.09
This is really good feature but according to Debjit there is maximum chance of hacking . so it need much more protection to the email account.
Written by xphunt3r on 05.31.09
Nice found amit. Just going to implement it!
But at the same time its having some negative points too because the same facebook account’s security is now dependent not not only on facebook but also on the particular email account’s security with which you have add this feature. If that email gets hacked then your facebook account could get in trouble too!
Written by Shahab khan on 05.31.09
Saha is right. Its too risky in case if our google or yahoo is hacked.
Written by Ramanathan on 06.01.09
does the user get signed off from google if he signs of from facebook. if not it can be some issue.. i guess this openid thing wont signout from the google account.. it just deletes the cookies or so..
Written by Aneesh on 06.03.09
Much required feature. Good to see.
Written by Rajesh Kumar on 06.09.09