Facebook already offers your contact information to Microsoft and Yahoo. However, I think friends who have emails set as "only me" or equivalent do not export. I tried the feature today and 10% of my contacts did not export to yahoo :/
Facebook would have to offer such a facility in order to allow users to opt-in, and there seems to be plenty of evidence that they do not want to do that.
We all know that Opt-in is like "does not exist", but they could at least say they are open.
edit: They could even sell it as a privacy feature ...