These are useful identifiers, but also protected information that few
systems are willing to divulge (for fear that the result will be more
spam for their users). A one-way-hash of an email address allows one to
do an email address query without divulging the email address. Of
course, this assumes that other social networking sites start divulging
this hash, too.
Do a web search for mbox_sha1sum to get started.
-Dale