Akamai Technologies, which delivers Internet video for many websites, said the inauguration was a record for them, with 7.7m people watching video streams at the same time, according to The Associated Press.
Keynote Systems, which tracks website performance, said the internet's top 40 sites slowed down by as much as 60% when the ceremony started, and many news sites saw even sharper declines in performance.
Major news outlets offered live feeds on their respective websites in what was potentially the most widely covered news event on the internet to date.
It was partly out of necessity, since many viewers were at work in front of their computers for the midday ceremony.
US citizens also submitted photos, videos, quotes, links and blogs for the site and uploaded photographs to be considered for the official inauguration book.
Microblogging site Twitter also had an official presidential Twitter stream -- normally @barackobama, but for the inauguration it was @obamainaugural.
Current TV, the user-generated TV network co-founded by Al Gore, combined a live-stream of the event with Twitter messages or "tweets." Messages from viewers played at the bottom of both Current's broadcast and webcast.