I don't think the existence of any one new service is a real threat to existing email services.
The people stuck on Yahoo and Hotmail seem to stay because that's the one email address they've always had. Some of them are teens/tweens. Some of the reason is because at least Yahoo had parental email address controls over the kids accounts. Getting hacked is just part of life for them. "S***,
I got hacked, don't click the links in my emails!" you'll often see beamed out from their Twitter accounts.
I landed on Gmail becuase it was easy to consolidate and manage my email from there. With forwards and Gmail's ability to check other addresses it made sense for me to move to Gmail.
I don't think there is going to be any death match. Sure, there will be some noise in the media for a while but after that facebook email will be nothing more than just another email account.