Roman has been sharing some more of the Windows Phone 8.1 SDK, and the latest is a look at how sharing works in apps.
It will work similar to Contracts in Windows 8, where apps will advertise abilities.
Here is what the Windows 8 SDK says:
Contracts A contract is like an agreement between one or more apps. Contracts define the requirements that apps must meet to participate in these unique Windows interactions.
For example, Windows lets users share content from one app to another. The app that shares content out supports a source contract by meeting specific requirements, while the app that receives the shared content supports a target contract by meeting a different set of requirements. Neither app needs to know anything about the other. Every app that participates in the sharing contract can be confident that the sharing workflow is completely supported, end-to-end, by Windows.
The system is more powerful and customizable, but one can of course see how it prevents the sharing to multiple social networks at once. On the other hands I do not think it will be long before a app comes along that will advertise the ability to share to multiple networks.
As another little unrelated teaser, it looks like Windows Phone 8.1 may have a file manager after all, with Roman saying:
I expect we will know more very soon.