In Facebook, people who view a photo can use their browsers to add tags that identify the people in the photo.
When you sync that photo’s album with iPhoto (which you do simply by opening the album in iPhoto), iPhoto reads those tags and passes them on to its Faces feature.
The next time you open that photo in naming view (that is, by selecting the photo and clicking the Name button), you’ll see something new: a label for that person’s face.
What next? To accept the label, click the Facebook logo in the label, fine-tuning ...