Actually it is not that hard to create a better product than Skype. Skype sucks in so many ways that i wouldn't know where to beginn. Exchange developers, exchange the designer and the UI team. Hate it - but use it every single day.
I would also here some experts opinions on this.
How do they compare on a higher level. Say a RESTful API with JSON+HAL, HATEOAS approach, Versioning, etc. the full thing for a bigger usecase like Twitter, Tumblr, Facebook, etc.
Comparing to Facebook and Twitter is a useless comparison, because their scale causes so many edge cases. If you have that kind of scale, you can afford to hire the best in every area and build everything from scratch. Better to compare to a typical medium-sized website in a similar sector - e.g. a blog like cuteoverload.com or ecommerce site like tigerdirect.com.
