Between 6 p.m. and 8 p.m. U.S. Eastern Time, the availability for the news sites from ABC, CBS, the LA Times and CNN Money dropped to almost 10 percent, meaning that about nine out of 10 visitors couldn't get the sites to load.
Starting at 5:30 p.m., the average download speed for news sites tracked by Keynote went from less than four seconds to almost 9 seconds, and their average availability dropped from almost 100 percent to 86 percent, the company said. News sites monitored by Keynote returned to normal performance and availability levels by 9:15 p.m.
Other news sites that experienced problems included AOL, MSNBC, NBC, the San Francisco Chronicle and Yahoo News, according to Keynote.
Meanwhile, Reuters' home page had a period of "significant slowdown" between around 6 p.m. to 7 p.m., according to Web monitoring company Pingdom.
Akamai's Net Usage Index for global news sites traffic surged at around 6 p.m. and hit a peak of 4.24 million visitors per minute at 6:26 p.m.
The usage spike also put a strain on online communication tools, like AOL's AIM instant messaging system, which went offline for about 40 minutes on Thursday.
"At AOL our AIM instant messaging service was undergoing a previously scheduled software update which should normally prove routine. It proved not to be. There was a significant increase in traffic due to [Michael Jackson's] news and AIM was down for approximately 40 minutes this afternoon," the company said in a statement.
Facebook, the world's most popular social-networking site, experienced an increase across its core actions, like the posting of messages, notes and links on member profiles, a spokeswoman said via e-mail.
"Status updates in particular saw an increase of more than 3x the amount than usual within an hour of the MJ news breaking and remained well above our usual numbers for several hours," the spokeswoman said.
Jackson died unexpectedly at age 50 from a heart attack in Los Angeles as he was reportedly getting ready to launch a comeback. One of the biggest stars in pop music history, on par with icons like Elvis Presley and the Beatles, Jackson saw his star fade in the 1990s. One of the most successful recording artists of the 1970s and 1980s, Jackson later became more well known for scandals including accusations of child molestation and controversies over his multiple facial plastic surgeries.