Earlier this week, Talking Points Memo announced that it had ended its relationship with Flipboard, the social newsreader that last month raised an additional $50 million in funding and boasts more than 50 million users. The reason? TPM's editor, Josh Marshall, says Flipboard simply isn't paying off:

We give them the entirety of our product — news stories, updates, posts, what-have-you — in exchange for a notional thing called exposure, brand awareness, blah blah blah and in theory or at some point in the future a cut of the ad revenues these services bring in for selling ads on their platforms. The problem is there are no ad revenues that go to the publishers. Where they exist they are literally trivial. The real payoff is supposed to be reach, letting new potential readers know we're out there.

Marshall goes on to say that while a larger audience is great, the fact that it might be limited to the Flipboard ecosystem isn't helpful when TPM makes its money from ads on its own site, not from ads on Flipboard. (Disclosure: I was an intern at TPM's Washington bureau in 2011.)

I suspect there's still a case for Flipboard. But the right metric isn't scale, or reach. It's about conversion — when a publisher like TPM is able to hook a Flipboard user into reading often enough that he or she winds up visiting the site outside of the app.

Marshall is far more skeptical than I am that that conversion ever happens. People who use Flipboard use it for a reason, he argues, and that reason makes it less likely that they will hop on their computers to visit TPM's homepage. What makes this calculation so challenging for publishers is that there's no good way to tell whether Flipboard is a replacement for Web browsing or a supplement to it.

That's not to say we don't have any evidence to go on. The Pew Research Center's 2012 report on the media industry found that nearly a quarter of news consumers use at least two devices to get their information. Almost a fifth use a PC/tablet combination. Users aren't abandoning older technologies, the study concludes. "Instead, their news experience widens and deepens."

If those industry-wide numbers hold for Flipboard users, at least some of them may be successful converts, leaping from TPM's content on the tablet to its content on the Web and vice versa. But to know for sure, you'd need a tracking solution sophisticated enough to be able to say, "This person began reading TPM on Flipboard and now, six months later, they're doing far more of their TPM consumption on the Web."

It's a technically (and ethically) difficult problem that requires tying various devices to the same owner. It so happens that advertisers are developing just these techniques. While they may raise new questions about consumer privacy, they could also help journalists learn new lessons about their industry.