Does anyone think it is possible to build an entirely new social network using Hype?
not impossible! but also not for the faint of heart. It would be a lot of work. But, please bear in mind. Hype (although it can be) is not at this current stage a solution in my opinion to any kind of web page building. SEO for one thing but there are other things which might become barriers when trying to build up a site.
I personally would go for an open source framework for a social network (Elgg, HumBub, Anahita, Stream to name a few) and make sure it’s compatible with Hype to be able to include the output HTML and JS.