This was the most watched inauguration is history. That is a fact. Previous record was not Obama (he had the largest On-The-Mall crowd in history 1.8 million), but Reagan with 42 million folks watching the televised version, was the previous record.
Trump's numbers came in at 30 million, beating out all previous inaugurations including Obama poor showing in 2012 (a little over 20 million) going back to Reagan. and this does not count social media. Nor does it count CNN's streaming video watched by 16 million nor Fox's streaming video coverage, not yet reported. 12 Networks and CNN's streaming video totals, gives us 46 million, along. Social media and Fox's streaming media will put the number watching this inauguration at well over 50 million.
You can go to http://ew.com/tv/2017/01/21/trump-inauguration-ratings/ for the source we will use going forward.