In the scope of a couple of days, several people — including Mashable staffers — have seen their Facebook cover photos disappear without explanation. The issue appears to be a move by Facebook to aggressively crack down on images that are considered promotional.

I first encountered the issue yesterday when Facebook ostensibly removed a promotional still from the TV series Doctor Who that I used as a cover photo. When I attempted to upload another image, I saw this message:

Pick a unique photo from your life to feature at the top of your timeline. Note: This space is not meant for banner ads or other promotions. Please don’t use content that is commercial, promotional, copyright-infringing or already in use on other people’s covers.

Since we published the original article about the incident, several readers have come forward, reporting the same thing happened to them in the comments. In addition, three other Mashable staffers reported Facebook removing their cover photos in the last 24 hours.

When asked if there was some kind of crackdown going on, a Facebook spokesperson told Mashable via email that Facebook's policies regarding photos and cover photos haven't changed. Facebook's terms of service specifies that a cover photo should be a "unique image that represents your Page."

SEE ALSO: How We Judge People by Their Facebook Cover Photos

The exact reason why Facebook removed each cover is a mystery, since the user is not informed, except by the glaring empty space where the photo used to be. It could be due to a copyright violation or that the photo was deemed to "promotional." Although Facebook removes the photo from the cover position, it doesn't actually delete the photo itself.

"Facebook is in business to make money," says Lou Kerner, a former social media analyst and founder of the Social Internet Fund. "The great thing about that is most ways they're going to make money is by letting people do what they want — as long as it doesn't break the law. For the most part, if they act in the user's best interest, they act in their own best interests."

While I speculated Facebook was removing cover photos to prevent the site from becoming too tacky, one of Mashable's commenters suggested Facebook was looking to preserve its business model. After all, if brands recruit "ambassadors" by encouraging — or paying — them upload promotional cover photos, that would detract from Facebook's own tools that are meant to help brands engage with their fans on the service.

Disney, for example, offers fans of its franchises images to download that are specifically formatted for Facebook Timeline. If this is indeed a crackdown, that practice could cease.

"That seems more heavy-handed than Facebook generally acts," says Kerner. "That sounds very egregious to me in terms of how they want brands and people to interact. I don't see how Facebook benefits by not allowing a brand's fans to engage with the brand like that."

How widespread is the practice? It's hard to say from the evidence so far, but based on Twitter reactions over the last day, it's definitely been happening regularly. Although some users say the removed photos were their own, the pattern that seems to be emerging is that the photos are either promotional or violate copyright: