Wouldn't adding hashed query strings to urls linking to sites outside of Facebook circumvent any effort put into containing de-anonymizing data? The wrong hash would lead nowhere, and the right one would give you away.

Facebook would only have to edit a specific field in users' posts' urls related to certain domains, so that different users access unique urls that link them to Facebook in a specific context.

Once outside the container, a user is traceable back to a Facebook profile.

I don't know if this is possible, but people don't really check urls, let alone human-unreadable ones.

I imagine that this could break navigation, but directing wrong queries to a default page would solve this. Anyway, they could still try their luck.