User A: jabber: email@example.com in their profile
User B: jabber firstname.lastname@example.org in their profile
if either user looks at the other's profile, the J button will be an xmpp URI.
User A jabber email@example.com in their profile
User B no jabber id in their profile
If user B look's at user A's profile the J would be a link as you have it now. If user A looks at user B's profile, then plazes.com should use the user's assigned JID user@plazes to send the message internally, through the plazes.com web interface and messages to user A should be routed externally to their client.