Facebook wants my phone number

Facebook has become the most popular social network in the history of known universe for a pretty simple reason: It appeals to our egos.

Our egos love to be connected, recognized and comforted. But those needs are generally tiny compared to our desire to be flattered. And one way Facebook continually flatters us is by asking for our phone number — continually. Like all the time.

But like any stranger seeking your digits, the site may have ulterior motives.

Ask Facebook, “Why am I being asked to add my phone number to my account?” and its help page will tell you this:

Adding your phone number to your account will help keep your account secure, make it easier for you to connect with friends and family on Facebook and make it easier to regain access to your account if you have trouble logging in.

That’s true. But are there other reason that it might want this piece of information — reasons that appeal directly to Facebook’s bottom line? Almost certainly. In fact, the business case for getting your phone number may be so strong that it’s likely at least part of the reason for the change in terms and conditions for WhatsApp, which is owned by the technology giant.

So what does Facebook get when it gets your phone number?

Potentially lots and lots of information about you — possibly even your favorite breakfast cereal.

Watch our chief research office Mikko Hypponen break down what the data scientists that help social networks sell ads learn about you from your number.

Even if you don’t mind being marketed at with ruthless efficiency, there may be other ways Facebook could use your number that you might want to consider.

You might have heard about the therapist who began seeing her patients pop in Facebook’s “People You May Know” module.

How did this happen?

Fusion‘s Kashmir Hill suggests that “an algorithm analyzing this network of phone contacts might reasonably assume all these people are connected.”

And in this case the therapist didn’t even remember giving her number to the site, but she had. If you’re logged in, you can check if Facebook has your number here.

This still could be some value to you in handing over your number. Two-factor authentication is generally a smart strategy for any account you want to protect — and you need to offer your smartphone number to access the SMS messages you’ll need to use.

But remember: If you make your number available on Facebook, people can find you by searching it.

So if you do use Facebook’s two-factor authentication, you should consider hiding your phone number for anyone but yourself.

To do this, go to your profile page, click “About” under your cover image and then in the left column click on “Contact and Basic Info”. Next to your mobile number, click “Edit” and select “Only Me”.

This will make sure strangers won’t find your number through your profile or vice versa. But it won’t stop Facebook from knowing what your favorite breakfast cereal is.