Facebook docs are quite sparse on this: http://developers.facebook.com/docs/reference/api/, see “Searching”.
It is possible to search against several object types, but it is not clear which specific object attributes / fields / columns (whatever you call them) are being searched.
Which specific attributes / fields are being searched for each object type?
from my experience with search api.
type=pages searches index of name and description
type=group searches index of name
type=event searches index of name and description
type=user searches index of name
type=post searches index of message
There maybe more included, these are just from my observation.