No -- Facebook's PHP code is translated to C++ by HipHop and compiled into a monolithic binary, and Cassandra has largely been deprecated in favor of HBase.
Didn't know they switched to HBase, it's still Java though.
"Facebook uses hphpi (and now hhvm) for day-to-day software development, but uses the HipHop compiler (hphpc) to create optimized binaries that serve the Facebook website. hphpc is in essence a traditional static compiler that converts PHP→AST→C++→x64."
According to wikipedia: http://en.wikipedia.org/wiki/HipHop_for_PHP
"in Q1 2013, the production version of facebook.com started running on HHVM, replacing HPHPc."
I wonder how well silverstripe runs on it....