I have searched and am not sure it is possible to create an app which will be in the list of options for the ShareLinkTask launcher. From what I have read, the only apps that can do this are built in apps.
The reason for leveraging this API is the following:
A user browses on the phone and would like to share this link via another social networking site. It is not Facebook or Twitter or any of the other built in sources.
Is this possible?