Now you can enter a friend's email address into the To: line when you send a message or share an album, and Facebook will email them the message. Your friends will be able to reply without signing up, and they will be able to see content you share with them. Keep in mind that all rules of privacy still apply; some Facebook content that you share (photos, groups, notes, etc.) won't be visible to your friend.
Facebook will add any email addresses that you message to your "friend selector", so you don't have to remember everyone's email address all the time. Also, if you message someone who is on Facebook but you use their email instead of their name, we'll automatically forward the message to their Facebook Inbox. As we continue to make Facebook more useful for everyone, these changes mean that there's no need to switch between Facebook and email for your daily communication needs.
PS. In case you didn't already know, you can invite friends to events and groups through their email addresses, and they can RSVP without signing up for Facebook. However, unlike messages, they will have to sign up to get in on the discussion boards and Wall posts.
Luke, a Facebook engineer, is never using email again. Ever.
Let the games begin...