Who's your friend?

That quiz, survey, or game you just did on Facebook: is it a harmless entertainment, a marketing ploy, or even a virus?

If you're like half the known world, including me, you're on Facebook. Maybe as a regular user, maybe someone with an account that is barely active, but just there. (If your profile pic is a blank silhouette, you know who you are.) Part of the appeal of Facebook is the low barriers to entry: it's full of applications, not produced by Facebook, that will do everything from ask you what Grateful Dead song you are or the five worst movies you've ever seen, to giving you the chance to "send" drinks, flowers, or other virtual trinkets to your friends (and, in the process, spamming said friends with pointless messages).

But there's more. First, the linked nature of Facebook's friends network means that applications can grow and spread exponentially, so a popular, or even a problem, application can spread through the whole network fast. Second, although all the apps give you a chance to "Allow" the application, the odds are that most users don't read the disclosure before hitting the "Allow" button.

Would it make a difference if they did? Maybe not.

Here are a couple of examples. There's a quiz that presents you with a set of questions about your health that will then tell you what your "true" age is. My snarky response to one of my friends was that it's pretty easy to determine your true age: subtract your date of birth from the present date and that will give you your true age, generally expressed in years.

According to today's Times, though, the quiz does more than present you with some tips to improve your health. You might not have thought about what happens to the answers you submit to the quiz, though, but the quiz makers did. According to the Times:

But while RealAge promotes better living through nonmedical solutions, the site makes its money by selling better living through drugs.

Pharmaceutical companies pay RealAge to compile test results of RealAge members and send them marketing messages by e-mail. The drug companies can even use RealAge answers to find people who show symptoms of a disease — and begin sending them messages about it even before the people have received a diagnosis from their doctors.

Yes, even though Oprah Winfrey's doctor friend Mehmet Oz likes it, and you can find out what Oprah's RealAge is, when you take the quiz you're turning over private health information to the drug companies. Do you really want that?

Or, take another example, PicDoodle. It looks like a regular Facebook app, and it claims that it enables you to make hilarious pictures of your friends. In fact, if you Allow the application it will create blank pictures of your friends, tag them, and cause the Facebook photos app to send your friends reports that they've been tagged, and they can see it if they allow the application. Before long, the virus is spread throughout Facebook.

Facebook claims it's not a virus, but I don't believe it.

So what's the message here? The same as we've been told about the whole Internet for years: be careful, know who you're dealing with, and don't give out any information you don't want to be made public.

Oh, and my "Real Age"? Fifty-five, but I'm expecting it to go up to fifty-six in a couple of months.