This week, Twitter began rolling out new email notifications. While Twitter has had notifications for things like direct messages, new followers, and Twitter updates in the past, the company tweeted that it was adding notifications for when someone you follow retweets or favorites one of your tweets.

The new notifictions may seem like a minor addition to Twitter’s services, but they do suggest that Twitter is relying on email to keep users engaged – at least on a more consistent basis. Colleen Taylor at GigaOm takes this idea a step further , saying it “could be a red flag that Twitter is trying to remedy a stickiness problem.” She writes:

Twitter’s strategy in this department has contrasted with how often other social networking sites such as Facebook send email notifications.

Since email notification technology is, of course, nothing new, Twitter’s decision to stay out of users’ inboxes has always seemed like a reflection of a greater company philosophy. Twitter Co-Founder Evan Williams told the New York Times  in 2007 that Twitter “adds a layer of information and connection to people’s lives that wasn’t there before” and “has the potential to be a really substantial part of how people keep in touch with each other.” In an interview with NPR earlier this year, Twitter Co-Founder Biz Stone said he “definitely [spends] way more time reading tweets than writing tweets.” By not sending a lot of emails for all these years, Twitter may well have been trying to prove a point: Receiving emails about Twitter activity would eventually seem just as ridiculous as getting phone calls about your inbox.

I don’t know about phone calls about your inbox, but phone calls to your inbox seem to be fairly well received. See Google Voice.

The inbox still reigns supreme as the premeire line of online communication. Though it may be cluttered with spam at times, it’s still where most Internet users go to check for important messages, and social networks – even Twitter and Facebook rely on it to some extent.

Of course additional email notifications aren’t the only thing Twitter is doing to increase engagement. The company just bought the popular TweetDeck Twitter client.