So is your argument then that there is so much vested in facebook in dealing with and optimizing for PHP at scale that the real cost would be acquiring that knowledge a second time on a different platform since you _will_ run into scale problems no matter what language you use ?

From my experience in dealing with PHP performance warts. You can make it somewhat fast, but man the insides are so messed up every time I get into the core and try to do anything I want to rip my hair out.