The addition of facebook username opens up another possiblity for emails. Typical web mail email systems like yahoo.com, gmail and hotmail allows both external (unknown recepients) and internal (from connections) emails and some of them (like Yahoo.com) allows the users to separate these email conversations between these two categories.
It will be easy for facebook to add a facility for its users to just send private emails (only visible to that person or a groups of persons) from right from the facebook interface. As a user will I like it, of course yes. Will I replace my email with facebook if they came up with this feature, not at all. Still this can be an interesting addition to facebook and can introduce a challenge to other email provides in terms of traffic.