A drop in email use is no biggie. If it's just about communication, people will IM, facebook message, tweet, SMS, whatever. No biggie. But email isn't just about communication; it's also about identity. And as far as that aspect, email will never go away. At least, I believe, not in my lifetime.

To put it simply, as long as you need an email address to sign up for Facebook, email will not go away.