It doesn't matter what you are marketing. Whether it's widgets, or coffee, or professional services, the reality is that one of your most valuable (and often under-utilized) weapons in your marketing arsenal is your email list. In fact, according to a McKinsey & Co study, email marketing is still 40 times more effective at reaching your target consumer, than Twitter or Facebook.

It's true, even as Americans complain about their email inboxes, email marketing is still one of the most effective ways to reach your audience. But that study is from 2014, which begs the question, is your email list still king?

The answer is yes, but the more interesting angle is why.

Here are 5 reasons that even more than twitter followers, or Facebook friends, likes, and followers, the most valuable tool in your social marketing toolbox is your permission-based list of email addresses.

1. Email has the greatest reach.

One of the most obvious reasons why is that there are roughly 3.8 billion people using email today. While Facebook, Twitter, Pinterest, and Instagram have grown their user bases since the study was published, for most businesses the chances are still better your target customer has an email address.

It's also far more likely to get to your audience since email inboxes aren't ruled by algorithms that decide what they actually see (although Gmail does separate out what it thinks are promotions but your audience can always still find those).

2. Email is customizable.

Email marketing gives you a far greater ability to create customized campaigns based on the interactions of your customers and potential customers. With marketing automation tools, you can create campaigns based on the type of contact (inbound lead, customer, referral, etc).

This gives you far greater control of your marketing and sales message than you can accomplish on social media alone. By personalizing and customizing your email interactions, you can increase the opportunity to close new business among your target audience.

3. Email is permission-based.

Sure, Twitter and Facebook are somewhat permission based. They both rely on people to "follow" or "like" your organization, but a follow or a like is far less of a commitment to you than a potential customer that gives you an email address. Understanding how to attract visitors to your site and create opportunities for them to convert to leads through email newsletter forms or content offers is such a valuable tool.

That person is asking you to provide them with valuable content and messages tailored to their needs. That level of permission is indicative of a much higher quality relationship than someone who simply clicked "follow" on Facebook.

4. Email is measurable.

Social analytics has come a long way, but the measurement tools for email are still far ahead of anything you can use for social media. Almost every email marketing tool allows you tell exactly who is opening your emails, what they do with them (click on your links), and what they do when they then visit your website.

Email also makes it easy to "test" different campaign messages, and measure the response, in real time.

5. Email is personal.

No matter what people say about their email inbox, the reality is that it's still the place where your potential clients communicate the most. It's easy to skip over a twitter feed or go days without checking on Facebook, but the average American checks their email as much as 80 times a day (according to some studies).

As a result, the reality is that email is where the message actually gets to your potential customers. The good news is that email marketing doesn't have to be complicated or scary. In fact, it's never been easier to effectively reach your customers through simple tools like Mailchimp or Constant Contact which allow you to create beautiful email campaigns that move your customers to take action.