I think being 'easier' in a Rails app is debatable. Facebook authentication certainly isn't 'hard'.

IMO OpenID is an awful user experience, I much prefer using Facebook. But that point is certainly debatable as well.

It has similar Allow/Deny UX as Facebook and opens you up to major email providers like Google and Yahoo as well as the plethora of other providers and ones such StackExchange if you're creating a technical site/app. Having only Facebook as an auth alienates 1/3rd of the US population.

It's mainly because of the data and open graphs yes. :) Facebook really provides a viral way of exposing my app to other potential users. The app is initially designed to evolve around Facebook but during the process, I decided to step away from that.

For my app it was being able to post to the timeline that made Facebook attractive as an auth system. Get more exposure quickly.

Yes, this is mainly the point. :)