Evidenced by:
>No threading to manage concurrency
R is used in production at EA, Activision, Ebay, Trulia, Google, Microsoft and many, many more. Those are just the ones I've seen give talks about scoring >1TBs regularly with R.
Every time somebody says R can't do be used for large data sets or is slow, I ask for more details and almost universally the programmer's complete lack of initiative is the weak link.