Why email is crucial to Google, Microsoft and Yahoo

I'll admit. I'm a sucker.

I have mail accounts to both Yahoo and Google.

Because Google.com just has a search box, I always enter mail.google... to save time.

But since Yahoo does have a rotation of stories (often very bad but I still sometimes get sucked in) I go to Yahoo.com first and then click to the mail section.

i always thought of email as a way to get you to get a username, which is just a "gateway drug" into other services the company offers. For example, all of a sudden it's easy to find yourself on the Picasa site, and upload pictures doesn't require ANOTHER username/password because you're already logged in (cookie)

So how does Alexa get these numbers, exactly?