[Dwolb]

In addition to wiseleo's comment, the key thing I think they got right was the weekly digest. It's never too frequent, it's relative, interesting, and fresh.

It was important to get the digest right because it granted Quora permission to keep getting user's attention even when away from the site (push vs pull) and also gave users a 'takeaway' - either information or topics of discussion to debate with friends through real conversation or an email forward.

This feature lets them have high retention rates and increases word-of-mouth to gain new users.

[inglor]

This so much. I usually unsubscribe very fast from these sort of emails - I accidentally left a checkbox marked when I signed up to read an article and it actually got me hooked. Lots of good material.

I would love to read an article about how to create such digest emails. Typically they are truly terrible but they managed to make it interesting.

[justhw]

Yep. I look forward to these weekly digests. Once I click a link hours go by quick.