Three options:
1. Create a name/email only contact in Gmail (on the web), then do a sync and link the gmail contact to the Facebook contact.
2. In People hit Menu then Add New and just add the name, then link it to the Facebook contact.
3. Set the People app to not display Facebook contacts (Menu, View, uncheck Facebook), then save the contact info the way you were trying (from a text). Then link to the Facebook contact. If you prefer to have your facebook contacts show in people, you can then turn that back on.
Note: in each of these, if you don't link to the Facebook contact and you display Facebook contacts in People, the contact will have multiple entries.