The idea is to get a better picture of second-screen behaviour and social TV activity, to correlate with more traditional ratings forms.
The data will be fully anonymised and privacy-protected, the companies said; Nielsen will collect demographics like age and gender in aggregate, not linked to a particular account, and the tracked TV shows will be given to Facebook as code numbers, making for a double-blind strategy.
Also, Facebook will not use the data to serve targeted ads, it told Digital Trends.
"We have worked with Nielsen under strong privacy principles," a spokesperson said. "We don't believe that audience measurement systems should be used to adjust targeting; they should only be used for measurement. This protects the privacy of people viewing ads and ensures that both advertisers and publishers have the same information about the audiences."