What problem would this solve?

If my problem is that I don't want Facebook to have the phone numbers in my address book, then surely I don't want them to have the SHA1 of each of the numbers in my address book, either?

Isn't it going to be easy for an organisation with Facebook's resources to build a dictionary of the space of SHA1s of phone numbers?

And if they salt the numbers, first, then they can't be compared with each other for suggestions? So whats the point?

Have I overlooked something?