It’s the old flames and not the new loves that really ring the sales bell for e-commerce companies.

A study of 7 billion online shopping sessions during the fourth quarter by experience marketing firm Monetate found Web surfers returning to a site became buyers much more often than first timers.

Overall conversion rates peaked during the fourth-quarter last year, hitting 3.48 percent, up modestly from 3.42 percent a year earlier, according to the study. But it was returning visitors who were the stars, converting 4.55 percent of the time, well ahead of the new visitor conversion rate of 2.49 percent.

Returning visitors were also more likely to add items to their cart, starting on the path to purchase 14.8 percent of the time. That’s nearly twice the rate of new visitors, who added something to their cart 7.61 percent of the time.

All of those clicks add up. Monetate said that during the quarter, visitors who were returning to a Web site made up just 48 percent of the sessions, but spent $5.3 billion — almost twice the $2.7 billion new visitors spent.

“Every business needs to balance acquisition and retention costs,” said Lucinda Duncalfe, chief executive officer of Monetate. “This struggle for balance leaves many marketers asking, ‘Where do I focus my efforts so they make the most impact?’ Of all the questions, the most critical to answer is, ‘Do I invest time and money acquiring new customers, or do I focus on retaining the customers I already have and personalize their experiences?’”

Duncalfe said brands could do both. “Customer retention and customer acquisition don’t have to be two parallel lines that never meet,” she said, noting brands can personalize the customer’s journal and improve conversion rates.

The study offers a digital version of a retail truism: that 20 percent of shoppers make up 80 percent of sales. It’s a dynamic that has long left store executives wondering whether they keep their core happy or go after new shoppers, who are expensive to bring in.

The issue is especially important online, with many growing e-commerce businesses still working their way to profitability and investors increasingly looking for metrics to gauge budding ventures.