- Agent: an abstract concept - I just prefer this term, but I think it's similar to James' Identity - for organisations, groups, bots, etc as well as people; specifically some entity with agency that can be tracked to the 'real world'.
- Persona: also an abstract concept - some version or subset of an Agent (*not* a subclass).
- Account: As associated with a particular service (as Sandro described).
- Profile: A document describing the holder of an Account and/or information about the Account (also as Sandro described). I agree that the identifier for the account is probably the URL for the profile. Which in the case of social networks tends to be a combination of the domain of the service hosting the account, and a user name (http://username.tumblr.com or http://twitter.com/username).
A link between an Agent and a Persona is *optional*, but could be one-to-one, one-to-many, many-to-one or many-to-many. And a Persona can hold many Accounts. Each Account has a Profile.
Explicit links between Agents and Personas could be necessary for some activites, eg. to collect your Google Adsense revenue, Google needs to know your 'real world' bank details, or when you invite someone to stay on Couchsurfing they need to know they can meet you face to face and have your address.