It’s mentioned in the documentation that one can reconstruct URLs to allow manually resending webmentions for missed backfeed. However, it appears this may no longer work(?) as these reconstructed URLs, which used to be static are now automatically redirecting to their siloed instances.
redirects to https://twitter.com/schnarfed/status/476408043819659264
Separately, though related, the example in the documentation for Instagram no longer seems to exist and could be replaced and the example for Google+ could be removed as the service no longer exists.