Facebook has quietly released a fix to an issue with the share link when updates were being made by 3rd party apps.
Previously, if you used a program like Post Planner or RSS Graffiti, your updates looked slightly different than manual updates, with the share link being in the actual update rather then next to the Like and Comment links.
Additionally, when someone would actually click the Share link to share the update, it would close their browser window after the share was complete, forcing them to reopen Facebook.
These two issues resulted in a very small number of shares overall.
With the new fix in place, the Share link is now next to the Like and Comment links, like it should be, and it no longer closes your browser window when the share is complete.