Crowds fill the National Mall to watch the ceremonial swearing-in of President Barack Obama. (J. Scott Applewhite/AP)

About 20.6 million people watched Barack Obama’s second inauguration Monday, Nielsen finally reported, two days later.

That’s a steep drop from four years ago, when Obama’s historic first swearing in attracted an average audience of about 38 million.

That first inauguration stands as the second most-watched in history, behind only Ronald Reagan’s first inauguration in 1981, which averaged about 42 million people.

While quite a plunge from his first, Obama’s second swearing in isn’t the least watched inauguration on record.

That distinction still belongs to George W. Bush’s second inaugural, which averaged 15.5 million viewers back in 2005, according to Nielsen stats.

This year’s numbers include viewing on ABC, CBS, NBC, Univision, Telemundo, Azteca, MundoFox, PBS, Fox News Channel, CNN Headline News, Fox Business Network, MSNBC, TV One, CNN, Current TV, CNBC, Centric and BET.

Nielsen’s numbers typically include live viewing and viewing off of DVR recordings played back later in the same day.

Nielsen’s numbers typically do not include office and school viewing, or online viewing.

The National Association of Broadcasters, catching sight of Monday’s numbers, boasted that nearly twice as many viewers tuned into broadcast television networks as chose cable news networks to watch coverage of Obama’s second inauguration.

The largest crowd was NBC’s, with 5.1 million viewers from 10 a.m.-4:35 p.m. ET.

Another 3.9 million viewers picked ABC during the ceremony, and CBS’s broadcast drew 3.7 million viewers.

Meanwhile, an average of 3.1 million viewers tuned to CNN, 2.3 million viewers were watching MSNBC, and Fox News attracted 1.3 million viewers.