In 2013, Nielsen announced the commercial launch of Nielsen Twitter TV ratings, a separate product that has nothing to do with their traditional TV ratings product.
Much confusion has ensued. Many people believe that tweets somehow are counted in the Nielsen TV ratings.
This paragraph from the release should clear some of it up:
Nielsen Twitter TV Ratings are a separate set of measures that complement traditional National TV Ratings. They do not change the traditional National TV Ratings.
Bottom line: Nielsen Twitter TV ratings are a separate product, and have no effect on the reported TV ratings that ad rates are based on.
In 2016, Nielsen expanded their social media measurement. It still has no effect on TV ratings. Read more about that here.