I was on the team at LinkedIn when we first wrote the thing on Ruby on Rails. Here's my writeup containing some more context:

http://ikaisays.com/2012/10/04/clearing-up-some-things-about...

While I'll freely admit v8 is much faster than MRI Ruby, the efficiency gains are likely more related to 1) the rewrite factor 2) moving to non-blocking 3) the fact that the original server ... um, needed love