Facebook’s login screen usually accompanies the email and password entry fields with a graphic depicting the social graph or a call to use Facebook’s mobile services. Recently, though, some users in the UK have seen Facebook using this highly visible space to promote third-party mobile network operator partners who are offering free data access to the Facebook for Every Phone mobile feature phone app.

In one example, it announced a “Special Offer from Three UK” and linked to the carrier’s Facebook Page. Placement on the login screen could make partnerships with Facebook more lucrative to third-parties. The site could one day run traditional Facebook ads on the login screen, but that’s unlikely since they wouldn’t be able to take advantage of demographic targeting since viewers would be logged out.

To our knowledge, Facebook has not done login screen promotions before. However, the page gets huge volumes of traffic but only requires half the space for login and signup options, making it a logical spot to show promotions.

The placement for Three UK and possibly other mobile carriers may have come as part of broader partnerships surrounding Facebook for Every Phone. The feature phone app provides users with deep access to Facebook functionality, similar to the site’s official smartphone apps. Facebook struck deals with 17 carriers around the world to give users 90 days of free data access through the app so they try it without paying. In return these carriers can try to upsell users on data services, and apparently be featured on the login screen.

>> Read more on Inside Facebook.