Android - How can I force-link a Google contact with a Facebook profile?

First, in the Facebook app, under its Settings, enable synchronization for all contacts.

Open up your contact list and select the Google contact you would like to merge. Then push the menu button and select "Edit Contact". Then hit the menu button one more time and select "Join".

Choose which contact you would like to merge it with and you should be good to go.

For those using Sense, you also can add the link manually (I found that Sense doesn't always add the notes tag to Google Contacts automatically). You can get the Facebook "id" by going to graph.facebook.com/username instead of www.facebook.com/username (i.e. after you click on a friend on the Facebook webpage, replace "www" with "graph"). Then just paste into Google Contacts notes:

<HTCData><Facebook>id:12345/friendof:67890</Facebook></HTCData>

where 12345 is your friend's id and 67890 is your id.

Sense picks up the link on the next sync with Google Contacts and the Facebook link is added permanently.