Facebook and my plugins is not an option, though, no. Sorry. Unity's internet access is super primitive: Send a URL, get a response. The end. Even if you just go to a url like http://mysite.com and your site then redirects you to http://www.mysite.com via a 301 redirect Unity will throw an error! Status 201 means "Everything is awesome after the redirect", Unity sees that as an error. Unity's web access is super simplistic: Call a URL, get back a page with a 200 status OR ELSE(!!!!!)
Thus, any type of AOUTH login that requires multiple calls to pages that send messages between servers before finally sending back a "Okay, all is good now, eventually"... yeah, I have no clue how to get that to work, unfortunately. I am thinking that will require writing an entirely separate system using non-Unity classes (maybe using sockets via some .NET native classs ??? I don't know). So, no, sorry, Facebook, Google, Twitter etc... all those OAUTH login systems are not supported by my kits. :(