Assume instead that “your idea is run-of-the-mill.” Assume that there are 10,000 other web-sites out there ... and that, no matter what you're trying to do now, you will not be (and therefore, do not have to be...) “the first.”

I fail to see how does that answer the OP question about Web-Development. The question isn't whether he found the next X-killer (where X == your social network of choice), but how to build a web application that will scale, otherwise he's building the next Twitter.