"Trump's 2017 inauguration ranks fifth in terms of television viewership: More people tuned in for the second inauguration of Richard Nixon, the inauguration of Jimmy Carter, the first inauguration of Ronald Reagan, and the first inauguration of Barack Obama."


"An analysis of news footage appears to indicate that fewer people attended President Trump’s inauguration than President Obama’s in 2009. The footage on this page was captured 45 minutes before each oath of office. Attendees were still entering the National Mall up until Mr. Trump’s speech."
"Public transit usage is also used to calculate crowd size. The Washington Metropolitan Area Transit Authority (Metro) reported that 570,557 Metro rides took place on 20 January 2017, a number little more than half the 1.1 million rides undertaken during Barack Obama's first inauguration in 2009, and still significantly lower than the 782,000 rides taken during President Obama's second inauguration in 2013. (These numbers measure the total number of rides recorded from the Metro's early morning opening through midnight, not just the number of rides occurring prior to the start of inaugural ceremonies.)"
"There are several problems with measuring online viewership. For one, there's no official body that tracks total views across all platforms. ComScore, a cross-platform Internet analytics company, told us that they did not have data on the inauguration's online viewership and that they weren't aware of any other company that would.
Another problem is that the way online viewership is measured is different than television. Web sites such as YouTube, for instance, measure the total number of people who click on a video but not the total number who view in its entirety. Nielsen, on the other hand, takes such factors into account before estimating total viewership.
Regardless of the problems inherent with measuring online viewers, Spicer stated that one network (CNN) carried nearly 17 million online streams of the inauguration, and when that total was added to the Nielsen numbers, it was clear that Trump's inauguration was the most viewed in history.
This claim, again, is problematic. CNN did not claim they had 17 million live online viewers for the inauguration; rather, they stated they experienced "17 million live video starts.""
"Furthermore, CNN recorded more live video streams (21 million) during President Obama's inauguration in 2009. While this factor certainly undermines Spicer's digital viewership claim, CNN is only one cable channel, and the numbers might be different if one were to consider another outlet such as the Fox News Channel with its Trump-friendly audience. (Those numbers are not available as of this writing.)
While Spicer claimed that Trump's inauguration had the largest audience in history due in large part to online viewership, that data simply isn't available. And the data that is available undercuts his claim."
"The "official" crowd size measurement for Donald Trump's inauguration is still unknown, but even President Trump's personal estimate (he guessed that 1.5 million people were in attendance) is below the 1.8 million mark estimated for Barack Obama's first inauguration in 2009."
http://www.snopes.com/trump-inauguration-viewership/