The theme in board meeting after board meeting in recent months is driving repeat usage and retention. Whether the product is a mobile app, a social app, a mobile social app, a marketplace, an ecommerce business, or something else, everyone is looking at montly actives/total registered users and saying "let's push that number up and to the right." And I'm nodding my head in violent agreement.

I asked one of the best product centric entrepreneurs I know why he thinks that Facebook does so well on that metric (monthly actives/total registered users) and he said to me "when I get an email from Facebook that a friend has tagged me in a photo, I click on it and go check it out every time." 

I've always thought that photo tagging was the killer feature and that photo sharing is Facebook's primary utility. I've said that on more than a few occasions. But there's another piece to this that you cannot leave out. That is the email you get that tells you that someone has tagged you and brings you back to Facebook.

I remember four or five years ago, people, myself included, were asking if social media was going to lead to the end of email as we know it. In an ironic twist of fate, it turns out that email is social media's secret weapon. And more and more social applications are leveraging the power of email to drive repeat usage and retention.

Our portfolio company Return Path is the category creator and market leader in email deliverability. I asked Matt Blumberg, Return Path's founder and CEO, yesterday if they are seeing an increase in the usage of email among their social media and mobile clients. He told me that they are seeing very strong growth across the board in that part of their client base. I'm not the least bit surprised. If you are going to start sending a lot more email, you need to make sure you do it right so that you don't just send all the mail into junk folders.

I do think the rise of alternative notification channels; sms, mobile push notifications, direct messages on twitter, facebook messaging, etc are going to move some of this kind of thing off of email over time. But today, if you want to drive retention and repeat usage, there isn't a better way to do it than email.

(For more from Fred, visit his blog)

(Image source: blog.hubspot)