What caught my eye was the implication that one can already be served via email. It appears that, yes, federal courts have allowed parties to be served via email in at least a handful of cases dating back to at least 2010, e.g. Snyder v. Energy Inc:
The title should read, service via Facebook not ruled defective in this case.
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.
That article seems inaccurate or at least incomplete. If you read the NY State decision [1] you can read the judge's reasoning. It seems pretty reasonable and is definitely not some sort of blanket rule that service by email is OK. Notably, the AOL mail service did send an automated "message read" receipt.
I think the idea was that the fact that the defendants were active on their Facebook accounts was sufficient to prove that they had in fact seen the Postings including the court service.
http://www.serve-now.com/articles/50/eservice