In 5.0, it looks like the correct REST call is to use the /people/userid/preferences (optionally with a filter for the documents or folders favorites). The documentation doesn't really make it clear how to add/remove favorites from a user's preferences. The original, 1.0 API docs that are pointed to in this post (which is a year old, I admit) indicate an explicit "favorites" call, but that doesn't appear to exist anymore.
Thanks for any help or links to samples!
-AJ