I just tried to sign up and have yet to receive a verification email (edit: it's been more than an hour). Why do all of the current generation of social apps s

I just tried to sign up and have yet to receive a verification email (edit: it's been more than an hour). Why do all of the current generation of social apps struggle with registration spikes? I've created systems that do a million new users in a day without breaking a sweat, and today's databases and hardware are capable of a few orders of magnitude higher throughput. The design and implementation was not particularly challenging. What are these things doing?