Frameworks, popular open source programs and prebuilt social api code.
.Net is great for building things from the ground and enterprise setups but when it comes to hooking into any social site MS just doesn't stack up. Even their sponsored Facebook framework was abandoned. They invested hundreds of millions into Facebook but couldn't keep the simple framework up-to-date for their developers.
No kid is going to say "Hey, I'd love to build an enterprise level reporting program with .net" Instead they're going to want to build cool plug-ins for popular open source programs. The next generation of programmers are growing up on PHP.