There are two main components to their site:

Promoting deals is the part that interests me most. What got me hooked on Groupon initially was getting a daily email from them.

Obviously, I'm a big fan of email. I want deals pushed to my inbox. Don't expect me to visit your website every day to see what's there. Most people won't.

Something interesting happened a few months ago: I added Groupon to my RSS feed and I discovered I was buying even more than I was before.

I realized it's all about the timing of these notifications.

The Groupon email is sent to me at around 5am, which means I read it when I wake up. But when I wake up, I usually have about 100 unread emails. In the morning I'm fighting fires, catching up on everything that happened since I went to sleep. I'm battling to get back to inbox zero.

So unless it's a really good deal, I probably won't get around to purchasing it. 10am just isn't a good time to sell something to me.

But the Groupon RSS feed updates at around midnight. That's when I'm online, catching up on Posterous work, and responding to email in real-time as they come in. It's the perfect time for me to get a small distraction like a Groupon offer.

I wonder if Groupon has A/B tested the timing of their outgoing email. If they sent them out at 5pm instead of 5am, would they see more conversion?

At Posterous, we've always believed in email notifications. We've historically run random tasks like sending emails at night but maybe that's the opposite of what we should be doing. A Posterous email might be the perfect distraction I need in the middle of my day.

You can try to optimize your features and product to death. But maybe all you need is well timed nagging.