If you added Facebook user with Farmville app to your Facebook friends, but you don’t have them as a Farmville neighbor, you can still visit his/her farm, get points for helping etc.

How do to that?
If user has Facebook ID 123456789
Link for his/her farm is
http://apps.facebook.com/onthefarm/index.php?visitId=123456789

You can get user’s Facebook ID by going to his profile page and clicking profile pic.
That will lead you to their album.
Check the URL in your browser. It should be something like
http://www.facebook.com/album.php?profile=1&id=123456789
That number after id= is Facebook ID

Replace whatever Facebook ID number you get with 123456789 and you are on the users farm.

You MUST be his/her Facebook friend to do that, you just don’t need to be Farmville neighbor