FTC Can Use Facebook to Serve Papers, Judge Rules
blog.upcounsel.com
blog.upcounsel.com
It seems that they would need some kind of proof of delivery.
http://www.courts.state.ny.us/Reporter/3dseries/2008/2008_28...
Generally, court rules have preferred methods of service. In unusual cases where the usual means fail, like this one, courts sometimes make exceptions to allow actual service via a particular method to count as formal service.