A user reported this already a year ago, with no effect:
The "Sharedaddy" module (v. 0.2.12) in Jetpack causes a "sharing_disabled" item to show up in the custom fields menu. This occurs on my WP install even when Sharedaddy is not enabled. Ideally, custom fields added by plugins should start with an underscore, so they remain invisible and don't clutter the interface.
Btw, are there any better feedback channels for Jetpack than this forum?