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Facebook is a personal vault that can contain photos of your firstborn, plans to bring down your government and, occasionally, a record of your indiscretions.

It can be scoured by police officers, partners and would-be employers. It can be mined by marketers to show tailored advertisements.

And now, with Facebook's newfangled search tool, it can allow strangers, along with "friends" on Facebook, to discover who you are, what you like and where you go.

Facebook insists it is up to you to decide how much you want others to see. And that is true, to some extent. But you cannot entirely opt out of Facebook searches. Facebook, however, does let you fine-tune who can see your "likes" and pictures, and, to a lesser extent, how much of yourself to expose to marketers.

The latest of its frequent changes to the site's privacy settings was made in December. Facebook is nudging each of its billion subscribers to review them. The nudge could not have been more timely, said Sarah Downey, a lawyer with the Boston company Abine, which markets tools to help users control their visibility online. "It is more important than ever to lock down your Facebook privacy settings now that everything you post will be even easier to find," she said.

That is to say, your settings will determine, to a large extent, who can find you when they search for women who buy dresses for toddlers or, more unsettling, women who jog a particular secluded trail.

What can you do? Ask yourself four simple questions.

QUESTION 1 How would you like to be found?

Go to "who can see my stuff" on the upper right side of your Facebook page. Click on "see more settings." By default, search engines can link to your timeline. You can turn that off if you wish.

Go to "activity log." Here you can review all your posts, pictures, "likes" and status updates. If you are concerned about who can see what, look at the original privacy setting of the original post.

In my case, I had been tagged eating a bowl of ricotta with my fingers at midnight near Arezzo. My friend who posted the picture enabled it to be seen by anyone, which means that it would show up in a stranger's search for, I don't know, people who eat ricotta with their fingers at midnight. I am tagged in other photos that are visible only to friends of the person who posted them. The point is, you want to look carefully at what the original settings are for those photos and "likes," and decide whether you would like to be associated with them "I don't get this Facebook thing either," said one woman whose friend request I had accepted in January 2008. "But everyone in our generation seems to be on it."

If you are concerned about things that might embarrass or endanger you on Facebook — Syrians who endorse the opposition may not want to be discovered by government apparatchiks — comb through your timeline and get rid of them. The only way to ensure that a post or photo is not discovered is to "unlike" or "delete" it.

Make yourself a pot of tea. This may take a while. The nostalgia may just be amusing.

QUESTION 2 What do you want the world to know about you?

Go to your profile page and click "About me." Decide if you would like your gender, or the name of your spouse, to be visible on your timeline. Think about whether you want your birthday to be seen on your timeline. Your date of birth is an important piece of personal information for hackers to exploit.

A tool created a couple of weeks ago by a team of college students offers to look for certain words and phrases that could embarrass other college students as they apply for internships and jobs. It is called Simplewash, formerly Facewash, and it looks for profanity, references to drugs and other faux pas that you do not necessarily want, say, a law school admissions officer to see. Socioclean is another application that scours your Facebook posts. It is selling its service to college campuses to offer to students.

QUESTION 3 Do you mind being tracked by advertisers?

Facebook has eyes across the Web; one study found that its so-called widget — the innocuous blue letter "f" — is integrated into 20 percent of the 10,000 most popular Web sites.

This is how it works. I browsed an e-commerce site for girls' dresses. When I logged back on to Facebook several days later, I was urged to buy dresses for "my darling daughter." Facebook says that this kind of "retargeting" is a lucrative source of revenue. If that is annoying, several tools can help you block trackers. Abine, DisconnectMe and Ghostery offer browser extensions. Once installed on your Web browser, these extensions will tell you how many trackers they have blocked.

If you see an ad on the right rail of your Facebook page based on your Web browsing history, you can also opt out directly on Facebook. Hover over the "X" next to the ad and choose from the drop-down menu: "Hide this ad," you could say. Or hide all ads from this brand. Facebook does not serve the ads itself, so to opt out of certain kinds of targeted ads, you must go to the third party that Facebook works with to show ads based on the Web sites you have browsed.

QUESTION 4 Whom do you want to befriend?

Now is the time to review whom you count among your Facebook friends. Your boss? Do you really want her to see pictures of you in Las Vegas? And the woman you met in Lamaze class: do you want the apps she has installed to know who you are? Privacyfix.com, a browser extension, shows you how to keep your friends' Facebook applications from sucking you into their orbit. It is preparing to introduce a tool to control what it calls your "exposure" to the Facebook search engine.

Secure.me offers a similar feature. Depending on your privacy settings, that photo-sharing app that your Lamaze compatriot just installed could, in one click, know who you are and have access to all the photos that you thought you were sharing with "friends."

One of Facebook's cleverest heists is the word "friend." It makes you think all your Facebook contacts are really your "friends." They may not be.