In April 2018, Mark Zuckerberg, Facebook’s chief executive, told Congress about an ambitious plan to share huge amounts of posts, links and other user data with researchers around the world so that they could study and flag disinformation on the site.

“Our goal is to focus on both providing ideas for preventing interference in 2018 and beyond, and also for holding us accountable,” Mr. Zuckerberg told lawmakers questioning him about Russian interference on the site in the 2016 presidential election. He said he hoped “the first results” would come by the end of that year.

But nearly 18 months later, much of the data remains unavailable to academics because Facebook says it has struggled to share the information while also protecting its users’ privacy. And the information the company eventually releases is expected to be far less comprehensive than originally described.

As a result, researchers say, the public may have little more insight into disinformation campaigns on the social network heading into the 2020 presidential election than they had in 2016. Seven nonprofit groups that have helped finance the research efforts, including the Knight Foundation and the Charles Koch Foundation, have even threatened to end their involvement.