Which one of Ruby-PHP-Python is best suited for Cassandra/Hadoop on 500M+ users?  I know language itself is not a big concern but I like to know base on proven success, infrastructure and available utilities around those frameworks! thanks so much.
Because Cassandra is written in Java, a client also in Java would likely have the best stability and maturity for your application.
As far as choosing between those 3 dynamic languages, I'd say whatever you're most comfortable with is best.  I don't know of any significant differences between client libraries in those languages.
Scability of Ruby-PHP-Python on Cassandra/Hadoop on 500M+ users