Audience ratings for online video used to be one of the Great Unknowns, as unfathomable as the ages of leading Hollywood starlets and the Illuminati membership roster. But leading research companies are starting to catch up. Nielsen, for example, just started using new network technology that lets it sample traffic from certain origination points like Netflix (NASDAQ: NFLX). And Nielsen and Rentrak, meanwhile, are gauging things like Twitter buzz to measure engagement. FierceOnlineVideo takes a look the latest advances in online video measurement here.