Millennials have come of age in the digital world, so email marketers have done a lot of hand-wringing about Millennials' inbox habits.

Email marketers' thinking has gone along the lines of, "Well, who would want to use a stuffy old inbox when there are flashy social alternatives instead?"

As it turns out, just about everyone.

Research shows that email is the most common digital activity among Millennials.

Not only that, email is the No. 1 way millennials want to keep in touch with companies, according to research from the Principal Financial Group.

That percentage probably isn't really a surprise to companies, but the fact is that there's a lot of time—and resources—being wasted on social media channels that could be better spent on email marketing.

It can all be blamed on one thing: FOMO.

The FOMO Effect

If you're a Millennial or manage a few of them, you've probably heard the dreadful acronym of FOMO (fear of missing out). FOMO is often caused by social media. People look at pictures of things their friends (or total strangers) are doing and wish they were doing it.

Marketers are just as guilty of FOMO. For example, any time another brand does something fun and exciting on Twitter or sees some crazy returns from Instagram, marketers suffer from FOMO.

That feeling leads to a lot of bad decisions and poor planning. Companies race to pour money into social media channels when it's still notoriously (almost comically, given the amount of money spent) hard to track ROI from social media.

A study found that, while 66% of B2B marketers and 44% of B2C marketers can prove clear ROI from email, just 17% of B2B marketers and 27% of B2C marketers can prove it from social media.

Despite the haze surrounding ROI, businesses spent $16.1 billion on social network ad spending in 2014. A Duke University survey found that CMOs plan to increase social media spend by 150% in the next five years—even though only 13% of marketers can measure performance from it.

All the while, email remains the best channel for ROI available to marketers today. Oh, and often offers the best, most accurate customer data.

FOMO vs. ROI

Email is an easy channel to overlook simply because it's a dependable engine and most brands are happy with what's in place. But now there are new technologies, like contextual marketing platforms, that are helping redefine how we think about email and interact with our inboxes.

Contextual email content that changes at the time of open depending on real-time data analytics (not complicated, cross-channel CRM integrations) allows brands to experiment with totally new ways of email marketing.

Imagine if you took a third of your social media spend and put it into email. You could spend more time and effort to build up your email list, instead of to manage a Twitter or Facebook community. You could buy new technologies that integrate with your existing email service provider and amplify the features and data analytics of your email marketing programs.

Companies are already doing this, from American Eagle and Gap to Best Western, Finish Line, and Nickelodeon. So, if you're so busy trying to prove ROI from social media that you're not rethinking email and experimenting with all the new technologies available, forget your fear of missing out on ROI from social—you're probably already missing out on all the potential ROI from email.