To link up the Facebook account with Yahoo users need to login to Facebook and go to Settings -> Account Settings. In the Account Settings page, click on to the “Linked Account” item. Users are given a choice of linking their Facebook.
Also you can try this i got from yahoo link website (http://help.yahoo.com/l/us/yahoo/pulse/socialnetwork/psnetwork-03.html) :
Here’s how to do this:
1. Go to your Yahoo! Pulse website.
2. Click Settings located at the upper-right corner the page.
3. Click Manage my linked accounts located under “Linked Accounts”.
4. On the new page that opens, click Link located at the right of Facebook under "Accounts You Can Link".
5. Sign in to Facebook by typing the email address and password that you used to sign up for Facebook.
6. Click Connect.
7. If this is the first time you’ve linked to Facebook, you’re presented with several pages asking you to allow specific actions to occur. If you agree with each request, click the Allow button.
Your link is established and you can immediately begin seeing your Facebook updates on your Yahoo! pages.
8. If you want to share your Yahoo! updates with your Facebook account, leave the box checked; otherwise, uncheck the box if you don’t want to share your Yahoo! updates with Facebook.
9. Click Continue to complete the linking process.