What if you were a model for a dating Web site - and you didn't know it?

Recently, a man from Virginia signed on to his Facebook page and was greeted with an advertisement featuring a woman's face. Above the photo were the words "Hey Peter. Hot singles are waiting for you!!" After a double take, the man realized that this hot single wasn't single at all. It was his wife, Cheryl.

Cheryl Smith was shocked to find out her photo was advertising a dating Web site. Her picture had been lifted without her knowledge by a company that had an advertising agreement with Facebook. Mrs. Smith refrained from taking legal action, commenting on her blog, "Good thing we have a sense of humor."

Sense of humor or not, privacy on Facebook and other social-networking sites remains tenuous and depends on the compliance of a host of largely obscure advertising companies. But is it Facebook's responsibility to make the network more secure, or should its users exercise more caution? It seems both sides could do better.

Given the number of third-party applications and companies operating with a degree of autonomy on Facebook, it's difficult for the massive site to ensure that its privacy policies are being observed. The case of the Smiths was unusual in that they discovered a potentially harmful theft of a photo that was in violation of Facebook's privacy policy. However, there are less severe and more routine forms of privacy invasion on the site.

For example, several weeks ago, I attended a lecture by the author Salman Rushdie and posted the information as a Facebook status update to share it with friends. Since that single mention, I have been bombarded with advertisements asking, "Like Salman Rushdie?" and suggesting I need to add the author to my "visual bookshelf." I don't plan on finding out what a visual bookshelf is, but the specificity of the ads remains unsettling.

If you are a Facebook user, take a look at the side of your screen next time you sign in. An ad might tempt you (with eerie accuracy) to buy a favorite band's CD or find out the specials at a restaurant you visited recently. These ads are tailored not to a demographic but, out of the infinite masses, to you and only you.

Facebook allows its users to be on the defensive by providing a minuscule "Thumbs down" or "Report" option next to all the ads. These safeguards help Facebook monitor ads that users find intrusive or inappropriate.

But Facebook's complicated and hidden privacy settings leave users in automatic agreement that their names and pictures can be used in Facebook-sponsored advertising, unless and until they locate a button allowing them to opt out. It should be the other way around.

MSNBC's Bob Sullivan, who warned readers about the case of the Smiths in his blog, provided a "click map" to show users how to exclude their private content from Facebook's ad domain: "Settings => Privacy => News Feed and Wall => Facebook Ads. Then select 'No one.' "

However, adjusting this arcane setting would not have prevented the "Hot singles" incident, since Smith's photo was accessed through one of Facebook's thousands of third-party applications, or "apps," which independently request permission to access users' photos and preferences.

On the surface, it's easy to blame Facebook for these application-specific indiscretions, as well as its own ad policy. However, a closer look reveals that the users bear much of the responsibility.

There is no feasible way for Facebook to control each application, other than cutting off those that blatantly disregard the privacy policy. So most decisions about third-party applications have to be left to users. Facebook requires each application to request permissions from users to access their information and run on their pages.

If a user indiscriminately agrees to access for quizzes, games, and the like, his or her privacy is significantly diminished. Users must review their settings under "Applications" to find out what they really agreed to when, for example, they took a quiz to find out "What '90s television show are you?"

Because Facebook and the third-party applications are free to users, many do not think of these online amenities as having a cost. But users should think of the use of personal information, access to profile information, and possibly even lifting of photos as a very real part of the costs of "free" social networking. In that light, a little caution and discretion on their part would go a long way.

Gina Tomaine is a senior majoring in English and economics at St. Joseph's University and a member of The Inquirer's Off Campus board of contributors. She can be reached at gina.tomaine@gmail.com.