CMS are mainly document-centric, whereas Hull is interaction-centric.
The social mechanics are available for those CMS as plugins, yes, but you need to work out how to integrate them, when they're made by different developers, and write glue code.
A big strengh of hull is also it's client-side widgets library, that just work and allow you to combine and extend them easily.
Lastly, hanging off OpenGraph does not make you the owner of the data.