Two more brands killed commenting functionality on their sites this week, joining a small but growing group of publishers that are instead leaning exclusively on social media for conversations with readers.

The Week announced that it would be eliminating comments beginning early next year, while Mic cut them off Wednesday. Both say their audiences are talking on Twitter, Facebook and other social platforms, more than they were at the bottom of their articles. Civility and the resources needed to keep comment sections productive come into play as well, they note.

The explanations echo those of Popular Science, Re/code, Reuters and others that have removed comment sections recently. Increasingly, they're being seen as a relic of the pre-social Web.

"We'd been talking about this for the better part of the year," says Ben Frumin, editor-in-chief of The Week. "We're a brand that, at its core, has smart arguments, insight, debate. [But] knowing the resources that it would take to really moderate our comment sections in a way that would help us achieve our goals, it made sense to acknowledge the fact that these conversations have moved off the bottoms of our articles and into places that didn't exist when we launched our comment section."

It boils down to ROI for many. Cultivating and moderating a vibrant community can be a big drain on resources, and on-site commentary often drives just a small percentage of traffic.

Frumin says TheWeek.com generated 68,000 comments in the most active month of the site's history. With 12 million unique visitors over that same period, commenters made up no more than 0.5 percent of all traffic (and, likely less than that).

Even accounting for lurkers—people who read comments, but don't leave their own—comment traffic is minimal. Less than 1 percent of all readers generally visit The Week's comment section, Frumin says, and it's "almost never more than 2 percent."

Cliff Ransom, editor-in-chief of Popular Science, was faced with a similar dilemma when the brand decided to scrap its comment section a little more than a year ago-one of the first major brands to do so.

"It's a lot of work to police a comment section," he says, noting that the group had two editors—half of its dedicated digital staff—spending a significant amount of time moderating threads. "Since then, there's been development of some technologies that help with the policing and automating that process, but it's still either going to be hard or require a lot of resources. We didn't have the opportunity to put that much into it. Unless we were to spend a lot of money and resources on it, a la [Gawker’s] Kinja, it's just going to be this artifact."

There was pushback initially, but Ransom says there hasn't been a material impact on traffic.

"It had zero impact on traffic," he says. "In fact, our best traffic month of 2013 happened to be September—the month that we turned off the comments."

Re/code came to the same conclusion in November, eliminating its comment section as one of a series of upgrades to its site.

Moderation followed a somewhat-stringent set of standards for legal reasons, so the barrier to entry was higher for potential commenters. Coupled with a social-savvy audience, on-site commenters made up just a small fraction of his site's readership, says Walt Mossberg, co-founder and co-executive editor for Re/code.

"Yet we knew that we had lots of vigorous discussion on Facebook and especially Twitter," he says. "It was a very stark contrast."

"We didn't drop comments," he continues, "we dropped on-site comments. That's a really important distinction. When the Web started, the place people thought would be most appropriate to put comments was on the site. Now, it's not. So it isn't like we were trying to get people to stop commenting on our articles, we just recognized where the commenting had gone."

Frumin, Ransom and Mossberg each point out that their decisions were made strictly for their brands, not as statements against commenting as a whole. They name sites like Gawker, Huffington Post and Quartz where commenting has changed and they play a central role in the way content gets consumed.

Like any community, comment sections can be valuable, but they need a lot of support, technically or from staff. And regardless of whether comment threads are more or less civil than Twitter, they carry a burden that just doesn't exist on social media.