I have the same problem. I have a Facebook contact, and a personal contact. They're the same person, but they used their middle name for Facebook. Whenever I want to join them, they won't show the list.
Mine works fine... I would just copy what ever # or info and paste it into one ID tag.. then delete the old one... Mine works fine though.. FYI the names have to be spelled exactly the same to get facebook to work.
I've also noticed that the linking with Facebook usernames doesn't happen immediately. If you change their name to mimmick their Facebook name, it took my Droid a couple minutes before they were linked up.
I do wish there was an option in the contacts app to manually link them up with a Facebook contact.