Facebook is reportedly planning to test a subscription offering built on its Instant Articles platform later this year.

But even as the world’s largest social network works to iron out details for the new paywall initiative, there are a number of challenges it will face that raise questions about its likely viability.

Here are six of the biggest that publishers should keep in mind as Facebook gears up to test the subs model.

Developing a workable subscription structure will be hard

Few specifics are known about Facebook’s Instant Articles paywall plans, but it seems clear that Facebook is going to face challenges developing a subscription structure that works for both consumers and publishers.

On one hand, it will likely be difficult for Facebook to build an offering that asks its users to subscribe to content from individual publishers. After all, with so many publishers active on Facebook, there will realistically be a limit to how many publishers users are going to subscribe to if they’re willing to subscribe at all. And logically, users who are already avid consumers of a particular publisher’s content are likely to already have a direct-purchased subscription.

On the other hand, if Facebook tries to create a subscription offering that bundles access to content from multiple publishers in a single subscription, it’s not clear how it would be able to generate adequate revenue for publishers. No matter how it splits the revenue (e.g. equally, based on consumption, etc.) publishers are likely to earn less and have less revenue stability than through their own offerings.

Instant Articles is already a monetization disappointment

According to reports, one of the reasons Facebook is launching subscriptions for Instant Articles is that many publishers have already been disappointed by Instant Articles’ ad-based monetization. In other words, despite Facebook’s prior attempts to keep publishers happy, ad-supported Instant Articles have failed to monetize as well as publishers’ own sites, so Facebook is adding subscriptions in an attempt to appease them and keep them on side.

This raises the question: if ad-supported Instant Articles don’t monetize as well as ad-supported articles published on publishers’ own websites, why would publishers expect subscriptions offered through Facebook to convert as well as if not better than their existing offerings?

Existing conversions data is not impressive

That question is an important one in light of the fact that some publishers have already discovered that Facebook users who interact with them through Instant Articles are less likely to purchase subscriptions when they land on their sites.

For example, the New York Times not only found that its content published on Instant Articles yielded less ad revenue than content published on its own site, but according to Kinsey Wilson, the New York Times’ EVP of product and technology, “people were also more likely to subscribe to the Times if they came directly to the site rather than through Facebook.”

In other words, it would appear, at least for the New York Times, that Facebook’s traffic has been less subscription friendly. Will a paywall erected by Facebook itself change that?

Some publishers have already reduced the amount of content they publish to Instant Articles

As a result of its experience with Instant Articles, the New York Times is one of a number of early adopter publishers that have since reduced their use of Instant Articles. In fact, as early as last year, the Times was reportedly publishing just a few pieces of content via Instant Articles on a daily basis and had “largely [retreated]” from it.

Another early adopter that went all-in on Instant Articles only to publicly retreat is The Guardian, and an analysis by NewsWhip seems to indicate that BBC News, National Geographic and the Wall Street Journal have also cut back on their Instant Articles activity.

While publishers like the Times and Guardian could obviously re-engage, convincing publishers to come back will likely be a tougher task than convincing them to give Instant Articles a try in the first place.

Instant Articles has arguably hastened the decline of publishers’ brands

One of the biggest reasons to doubt Facebook’s ability to convert its users into paying subscribers comes in the form of a newly-published Reuters study which found that less than 50% of UK news consumers who found a news story through social media could remember the publisher who published it two days later.

Instead, they were far more likely to remember the platform through which they found an article.

As study authors Antonis Kalogeropoulos and Nic Newman wrote, “The finding that people are more likely to remember the platform where they found the content (e.g. Facebook), rather than the news brand that created the content, will be troubling for many publishers.”

There is a caveat: some strong brands were frequently recalled, particularly by social users who were part of their core reader base. According to Kalogeropoulos and Newman, “This suggests that low attribution in distributed platforms could be more closely related to weak levels of pre-existing engagement than the impact of the platform itself.”

But even here the researchers noted an additional caveat:

Having said that, it could be argued that the weakness of many existing publisher relationships with consumers is partly a consequence of the shift to the discovery of content via third parties and the amount of time spent with platforms like Facebook.

Bottom line: publishers relying heavily on third-party social platforms for distribution are taking on additional brand risk and subscription offerings that they participate in on these platforms could only increase the risk.

Facebook’s offering could lack content from the most desirable publishers

Because of all the challenges and risks associated with an Instant Articles paywall, it’s possible that publishers, especially those that have successful subscription offerings of their own, might choose not to participate, forcing Facebook to launch its new effort with publishers that don’t have the type of content that is most likely to convince consumers to open their wallets.

Already, a New York Times executive revealed that while the publisher is still considering whether or not to participate in Facebook’s initiative, told Digiday, “So far, we don’t see any great upside.”

If publishers with a proven ability to get consumers to pay for content aren’t on board with Facebook’s new offering, it could put the company in a doubly tough position on Day 1.