This won't be the popular answer, but Wordpress is actually pretty good for prototyping things like this. The plugin hooks system makes it easy to try things ou

This won't be the popular answer, but Wordpress is actually pretty good for prototyping things like this. The plugin hooks system makes it easy to try things out quickly. There is already a plugin for just about everything, so adding Facebook Connect, etc. can be done in a couple minutes. I actually gave a talk at a local WordCamp about this. I built a couple prototypes that had APIs via simple plugins and leveraged WP users and posts (with custom post types) for content. And nearly everything can be abstracted out to the point that users won't know it's running on Wordpress. As far as what YOU should use, the correct answer will always be "whatever you're comfortable with."