Actually, that would be great for facebook. Sending millions of emails per day to the outside world with a link 'XXX is already on facebook, why dont you join today' at the bottom would be priceless.
Well, yes and no. A major advantage of FB is that, once they suppressed the crazy app invites, it's spam-free. The only people who can send me messages are people I've already "whitelisted" by adding them/accepting their add. Once they start allowing inbound email, it all goes a bit hatstand.
