San Francisco (CNN) -- iSad. That was the reaction of many as millions took to the web with the news of the death of 56-year-old Steve Jobs.





Facebook and Twitter messages were filled with links and anecdotes about the impact technology -- and more specifically, Apple -- had on users' lives.





Apple invited people to email their thoughts, memories and condolences after announcing Jobs' death Wednesday, saying the company lost a "visionary and creative genius and the world has lost an amazing human being."





CNN's iReport is also collecting reactions.





Jobs' death jolted a generation that has never known a world without a cellular phone, and it elicited reactions from those who watched Apple grow from a Silicon Valley garage startup in 1976 to today's leading tech company.





"RIP, Steve Jobs," read many Facebook messages. Still others invoked the visionary's signature stage pitch, "And one more thing ... Thank you."





Many tweeted simply: iSad or iHeaven.





"Oh my God, dad. Steve Jobs has died. He's my generation's Walt Disney," 20-year-old Lauren Harrington of Atlanta told her father upon learning Wednesday night that Jobs had died.





Celebrities, corporate executives and politicians, including President Barack Obama, paid tribute to Jobs.





"Our parents had JFK, we had Steve Jobs. Edison gave us electricity, Jobs gave us the Jetsons in real life," tweeted director and actor Kevin Smith. "We lost an icon today. Mourn him."





Actor Ashton Kutcher tweeted: "Sending love & light to everyone @Apple & the entire Jobs family. Today we lost a Giant who will be missed even by those who didn't know him. I never thought I could be so busted up about the loss of someone I never met."





Obama hailed Jobs as one of America's greatest innovators, a man "brave enough to think differently, bold enough to believe he could change the world, and talented enough to do it."





"The world has lost a visionary. And there may be no greater tribute to Steve's success than the fact that much of the world learned of his passing on a device he invented," Obama said in a statement released Wednesday.





Obama, an avowed BlackBerry fan, revealed this week to ABC's George Stephanopoulos that Jobs personally gave him an advanced copy of the iPad 2.





House Majority Leader Rep. Eric Cantor (R-Virginia) tweeted: "There is not a day that goes by, and often not an hour, that a Steve Jobs' invention does not better my family's life. Thank you Steve."





The flood of messages slowed Twitter to a crawl at times or produced error messages saying the site was over capacity.





On the social networking sites, many quoted from Jobs' heartfelt commencement address at Stanford University in 2005, where he first detailed his battle with pancreatic cancer.





Actor Hill Harper, paraphrasing the speech, tweeted: "Do what you believe is great work. And the only way to do great work is to love what you do! ... Don't settle."





Apple flew the flags at its Cupertino, California, office at half-staff, and an impromptu memorial sprang up at the venue as people arrived with flowers, letters and mementos, CNN affiliates reported. Several employees tweeted messages containing only the Apple logo.





The many websites devoted to Apple rumors and products placed large banners in memory of the company's co-founder.





The alternative-culture blog Boing Boing revamped its design in honor of Jobs to resemble retro Macintosh software. Wired painted its front page black, with a shadowed picture of Steve Jobs in the center.





Google's co-founders posted statements about Jobs' importance to them personally and to the industry, and underneath the search box on Google.com, the company added: "Steve Jobs, 1955 - 2011."





Research in Motion, from its BlackBerry Twitter account, called Jobs "a great visionary and respected competitor."





Microsoft's Bill Gates, who once worked with
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