I tried out Wattpad and also surfed the website - just to see how the content differed. The Wattpad app more readily offers up content for reading while the website makes its social aspects more apparent. I think a lot of the appeal of Wattpad is that it lets everyone be an author. There are community forums for proofreading, suggestions and just hanging out. It is also a nice way to access many of the Project Gutenberg texts. And if you like reading fanfiction this is a fantastic place to be.
Wattpad has also helped launch many writing careers that had stalled out in the traditional publishing world and has connected new, young writers with audiences (and eventually agents). Interesting!