If you’ve put embarrassing photos or posts on Facebook, you’ll have seven days after switching to Timeline to prune them from your profile before Timeline becomes the default look on your profile page.

Facebook CEO Mark Zuckerberg has called Timeline a way to share life experiences, but it could also become a carrot for marketers and advertisers to target consumers based on their “likes” and devotion to certain brands as Facebook duels with Google+ and other social networks for advertising dollars, analysts say.

The old Facebook profile page shows the most recent items users have posted, along with things like photos of them posted by others.

But Timeline creates a scrapbook-like montage, assembling photos, links and updates for each month and year since they signed up for Facebook.

When Zuckerberg, introduced Timeline in September at a developer conference, he described it as a way to get a more comprehensive portrait of someone than by simply reading updates or looking at a profile picture: “We think it’s an important next step to help tell the story of your life.”

Facebook said in a blog post that users could either wait to receive a notification about Timeline on their pages or go to facebook.com/about/timeline to activate it immediately.

“It’s unsettling to see the past presented as clearly as the present,” Mr. Werdmuller said. “It’s your life in context, all in one place.”

Several hundred Facebook users shared their initial reactions to Timeline on the company’s blog post.

While many appeared to be the kind of denouncements that are generated by any tweak to Facebook’s site, a large percentage welcomed the changes.

“A treat for profile stalkers,” wrote a Facebook user named Mudit Goyal.

Another, Joshua Bamberg, said, “If Facebook didn’t change stuff every couple months, we would still be using MySpace.”

And Tatsat Banerjee wrote: “Now our Facebook profile is almost equivalent to a personal Web site. Make no mistake, this is the best update Facebook has ever done till now.”

Analysts say Timeline is a significant evolutionary shift for Facebook. For starters, linking Facebook more closely to memories could make it harder for people to abandon the service for rivals.