I was thinking more along the lines of having unique emails per site. While usernames have a higher probability of being the same person across different sites,

I was thinking more along the lines of having unique emails per site. While usernames have a higher probability of being the same person across different sites, it's not guaranteed. twitter.com/ed may not be facebook.com/ed but foo@bar.com is definitely unique. The goal would be to make email one less piece of PII to aggregate you across sites as well as helping narrow down who is selling your info. I am well aware of the <username>+foo convention of gmail but it is trival to strip off the +foo part. This way, you still get mail that you want without having to give out your "real" email address. Maybe it's wasted effort but personally, I think there is some value there (for the people that care).