The biggest and most battle tested implementation is certainly Erlang OTP and not Akka. I'm actually happy more attention will be given to ol' boring Erlang now.
I guess I shouldn't have said that without checking, I'm not sure who's on top. My point still stands though. Most dev shops are already operating on the JVM, trying out akka was a much easier sell than adopting BEAM. Whether or not akka is more widely deployed than erlang, IMO akka was driving more adoption of the actor model.