So what if your emails wind up in prospective customers' spam folder? That's no reason to give up on the marketing strategy, reports Custora, a predictive analytics platform for e-commerce retailers.

In fact, customer acquisition via email has quadrupled over the last four years, leaving social media platforms like Facebook in the dust.

In examining the percentage of customers acquired via different marketing channels from 2009 to 2013, Custora found email marketing went from attracting 0.88 percent of customers in 2009 to attracting 6.84 percent in 2013.

Facebook, which drew less than 0.01 percent of customers in 2009, now draws 0.17 percent of customers -- not even a quarter of what email garnered five years ago. As for Twitter, the microblogging platform draws less than 0.01 percent of customers. 

The rise of email as a successful marketing tool could be attributed to the fact that not all of Custora's clients use social media to lure their customers, says Aaron Goodman, lead data scientist at Custora. In fact, in a study conducted last year, Forrester Research found social media accounts for only one percent of sales. 

What's interesting is how valuable a customer might be based on how they discovered a brand. Custora bases that value on the profit a company is likely to derive from a customer during the course of their relationship. 

"Customers who are acquired through Twitter are attracted to the promotions" often found in promoted tweets, says Goodman. In contrast, customers who willingly give their email to an online retailer are "more interested in the brand." 

It's no surprise, then, that the value of a customer acquired by email is 12 percentage points above average and the value of a customer acquired by Twitter is 23 percent below average. 

The most valuable customers come through organic search, says Custora, accounting for 15.81 percent of customers, but those customers's value was 54 percentage points above average. 

Custora evaluated data from 72 million customers from 86 U.S. retailers across 14 industries.