The term "resource owner" is defined in the OAuth v2.0 Specification, as "An entity capable of granting access to a protected resource. When the resource owner is a person, it is referred to as an end-user."
My question is, when is a resource owner not an end-user? I would appreciate explanation through examples that could be real use cases. For example, if the protected resource is a Facebook user's photo, is the resource owner Facebook or the Facebook user who uploaded the photo? Also, why is the resource owner (that is also a person) be considered an end-user if that person is not even a user of the application that is implementing OAuth? And, if the Facebook user is the resource owner, then what role does Facebook play in this exchange?
Resource owner can be a machine, not just people. There are many cases where no humans are involved in the entire OAuth flow, especially in enterprise setups. At least, that's what I meant when I introduced the term in RFC 5849 (and later in OAuth 2.0).