Well said. If I had to scale for tens of millions of users, 80% of which log in at least once per day, I would be focusing my effort on the most EFFICIENT tech

Well said. If I had to scale for tens of millions of users, 80% of which log in at least once per day, I would be focusing my effort on the most EFFICIENT technologies (watts per request). If there's capital, I'd look at what Facebook has done already and give it my own twist. I'd purchase the property and hardware up front and get to work building the platform, forking existing code where needed. If we're talking about 10,000 users, then I'm going to pick a technology that gives me a lot more flexibility. In fact, I would probably just extend an off-the-shelf PHP/MySQL package with an nginx EC2 instance to save myself the effort of building from scratch, because I wouldn't have to.