A few days ago, at the recommendation of @cgutteridge, I changed the default FOAF class from foaf:Person to foaf:Agent, when representing Twitter users.

As Christoper rightly pointed out, foaf:Person is used specifically to represent real-live people, whereas many of the accounts on twitter are either organisations or bots of various kinds. As such, foaf:Agent is the more appropriate class.

It would be great, of course, if Twitter allowed you to specify whether the account was for a real person or some other entity, but doesn’t (yet) allow you to do so.