A word of caution on this as a business, though: this is going to be brutal. You're selling a service that every competent web developer can build. You can save them time -- but not a huge amount of time, especially compared to the prepackaged user registration apps that already exist for common web frameworks. And the customers that grow really big will have strategic reasons to want to move off of your service, in addition to any cost-saving reasons. Maybe over time you can grow into something like http://www.gigya.com/ , which powers registration & social/gamification components for mainly content sites. But to do that you'll need to find a market of people who aren't developers, for whom your thing is a real enabler, not just a time saver.
TL;DR: Unless you're building something technically complex enough that everyone realizes they can't come close to replicating it, don't build stuff for developers. We're stingy and we think we can do anything ourselves in a weekend.