Update April 3rd: As per this LL blog post, the Facebook photo upload has been re-enabled by Facebook.
SL Share is the viewer-side capability which allows Second Life users to share pictures, thoughts, etc., with their Facebook account.
Since its launch, it has proven extremely popular among SL users who have no issue in linking their SL and RL identities, and who see it as a means of telling friends about their SL activities. However, a recent spike in usage of the capability highlighted the fact it is in violation of Facebook’s policy by posting SLurls with images sent to Facebook.
As a result, the Lab has announced that the photo upload capability within SL Share has been disabled by Facebook, and the Lab are now working with Facebook to resolve the issue.
The Lab’s blog post announcing the situation reads in full:
Facebook recently contacted us to let us know that the Photo Upload feature of SL Share is not permitted to automatically include location SLURLs in posts made from the application. We’re working with them to get a hotfix out ASAP, but in the meantime the Photo Upload feature in SL Share will not work, as Facebook has temporarily disabled that part of the application. SL Share’s Status Update and Check-In features will continue to work.
When SL Share’s full functionality is restored, SLURLs will no longer be included when you share a picture using Photo Upload, but you will still be able to let your Facebook friends know where to join you in Second Life by using the Check-In feature.
We apologize for the inconvenience this may cause you and are working to get a fix out ASAP. We’ll use this blog to keep everyone posted with any updates and will of course let you know once the issue is resolved as well. Thank you for your patience.

The situation likely means that any TPVs what have also included the SL Share capability into their offering will need to adopt any hotfix forthcoming from the Lab once it has been made available.