First, it's really hard to do a large rewrite of software. Netscape, for example, made the mistake of trying to rewrite their browser from scratch, and it killed them [1].
Second, what evidence to you base your statement "hiring talent at FB has to be getting harder"? If you're talking about market cap / upside, Google has had no problems hiring (well, relative to other tech companies, anyway), despite a market cap 3.5x+ what FB is today.
Lastly, FB is not all PHP. A lot of services are written in C++ or Java, and use Thrift to talk to the frontend [2]. Thrift allows a more gradual movement off PHP, by moving isolated components to other languages individually.
Second, what evidence to you base your statement "hiring talent at FB has to be getting harder"? If you're talking about market cap / upside, Google has had no problems hiring (well, relative to other tech companies, anyway), despite a market cap 3.5x+ what FB is today.
Lastly, FB is not all PHP. A lot of services are written in C++ or Java, and use Thrift to talk to the frontend [2]. Thrift allows a more gradual movement off PHP, by moving isolated components to other languages individually.
[1] http://www.joelonsoftware.com/articles/fog0000000069.html [2] http://www.quora.com/Facebook-Engineering/What-is-Facebooks-...