Facebook on Friday begins rolling out its biggest ever shake-up, in a move observers say will not only profoundly alter how its 800 million users interact with the site, but will keep them coming back for decades to come.

The new "Timeline," revealed last week by chief executive Mark Zuckerberg will also likely shock some users, notorious for backlashes in recent years over even small adjustments to the site, let alone a complete re-think of how their lives are presented to the world.

The changes amount to the "heart of your Facebook experience, completely rethought from the ground up," Zuckerberg told an annual developers conference.

Rick Marini, CEO of the Facebook-focused "career network" BranchOut, which itself boasts millions of users, marveled this week in a conference call with marketers and reporters how the site had managed to build an enduring model.

"If your Timeline becomes an important part of your life -- the diary of your life -- Facebook may have just locked people in for the next 20 years," Marini said Wednesday.

If Facebook is "where all of this happens, all your pictures, all your video, everything you've ever done," he added: "you're never going to leave."

Zuckerberg said his team's main aim was "to design a place that feels like your home," prompting Forrester analyst Sean Corcoran to note that Facebook was "positioning itself as not just your social graph online, but your life online."

Pete Cashmore, founder of influential tech blog Mashable, this week anticipated the expected response from users in an article for CNN.com under the blunt headline, "You'll freak when you see the new Facebook."

The move, Cashmore said, would be likely jarring at first for millions of people who visit the site every day.

But when they see their lives laid out before them in a neat, single page, they will realize Facebook has "unleashed something so remarkable that you didn't even recognize it at first: A meaningful social network," he wrote.

Observers saw a clear challenge to ubiquitous search giant Google, as 27-year-old Zuckerberg explained that the site he launched from his Harvard dormitory room in 2004 sought to be a central hub for the Internet.

The overhauled "Timeline" profile pages, as users will see as it opens up to the world Friday, with access further expanded in coming weeks, shows that the social network behemoth has saved everything that's ever been uploaded.

Mapping people's lives -- through photos, videos, status updates, comments, and even "likes" of specific content -- the new Facebook catalogues it all, literally from the beginning: scrolling to the bottom, every user's Timeline kicks off with an entry for their birth.

"Timeline is the story of your life," the young CEO announced last week with a barely contained sense of glee.

Yet apart from a relatively small number of industry geeks who followed the Zuckerberg announcement last week, the vast majority of 800 million users have little idea of the extent of the changes afoot the next time they log on.

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