Not sure why you got modded down because it's essentially true and I took their post to be alluding to as much. My intuition tells me that though they are huge, they aren't remotely in the same area as AIM, MSN, GMail, Yahoo Mail, etc. Twitter hangs right in the middle of being an instant messaging and email service.
I'll leave the armchair architecting to others.
But we've had e-mail working fine for years, and Twitter is just e-mail.