Facebook users invitions by e-mail

Question

I have a problem - need to add to site friends invite in style, as use this Youtube and Skype . Both of this networks has users e-mails, but havent associate every userID`s in Skype/YouTube with they e-mail. But both of networks have 3 things:

1. Friends from external IN they network , but not friends on current network
2. Friends from external IN they network and friends on current network (its not big problem, if i known e-mails)
3. Friends from external NOT IN they network , but friends (contact) on external network.

1 to 1 associate can do only with e-mail, i think For Google.Contacts - its not problem - contacts have mails, but HOW DO IT for Facebook?

PS. Sorry for bad english - havent nice skill