Who to specify in rel=author?

Question

Search Engine Optimization

Previously, the only recommendation I'm aware of for this tag is a Google+ account.
Google+ is shutting down soon.
What should be indicated there now?
Does this tag have any real use at all (signaling original content to Google, used to be rich snippet - what else?)

Answer 1

It is unlikely that this tag is somehow important for search engines. The maximum why it is needed is to tell the parsers who to indicate as the author :)