How does Facebook suggest friends?

Facebook's "people you may know" suggestion can show us some interesting people.  Why is it showing a random person you have never met, lives in a different continent, and does not speak the same language as you?  Why is it suggesting that random one-night stand from St Tropez?  Why is it suggesting your science teacher from school?

Facebook does not randomly suggest friends.  There are a number of ways Facebook suggests friends due to the unique way their algorithm works and here is a simple breakdown of the "people you may know" process.

Friends of friends

The most obvious source of friend connections is your current friends. Facebook will zone in on friends with whom you have the most mutual friends.

Additionally, Facebook will attempt to connect you to others through your online actions such as likes and dislikes, comments, and who else is in a tagged photo featuring yourself.

Friends from school and work

School, university, place of worship, favourite shopping centre, or work friends are an easy way for Facebook to connect you with others (even if you were never friends with them).  You can guard your privacy and not list any school or university information on your page but if your sibling has listed their school, Facebook’s scrupulous algorithm will make the connection.

If you do not record your university or work and do not have any siblings or spouses then Facebook will predict the university or work you most likely attended from analysing your friends school or place of work.  If 80% of your friends studied at Cardiff University or 75% of your friends work for KPMG, then Facebook will suggest people who have listed the same university or work and are of a similar age to you.

As you may have discovered, this makes it hard to hide from that weirdo in your school year group.

Your contacts list

Your phone and e-mail contacts are gold dust for Facebook.  If you use Facebook on your mobile phone or other smart device, Facebook will access the phone numbers in your contacts list and suggest those who are not on your friends list.  Even if you have not synced your numbers with apps, if you or your friend has provided a phone number as a verification number for a Facebook log-in then Facebook will use that number to connect you with people who also have that number.

Using the Facebook app on your smartphone allows it to access all the contact and lifestyle information to suggest friends.

If you have used e-mail or phone numbers on your phone or other smart device, then Facebook will use that information for suggested friends it finds with those contact details.  Lots of people I do business with pop up as friend suggestions but we have no mutual friends.

Additionally, I had a client's number on my phone and a few days later she popped up in a "Do you know..." e-mail.  It may is assessed I have appeared in hers if she has her Facebook registered with the phone number she gave me.

If you want to completely opt out, you need to take your phone number off Facebook.  Bear in mind your phone book will be continuously synced so any new numbers you add might show up in "People You May Know".

Stalking users

When you view a Facebook profile via the Facebook app, Facebook takes a data snapshot.  If you regularly visit a specific person’s profile but have not friended him yet Facebook will show him up as "someone you may know" until you virtually friend him.

Facebook's intelligent data crawler is always monitoring users actions and will suggest a person who is regularly looking at your profile but not a friend (yet)!

Other Facebook users on the same WiFi connection

Many travellers will connect to the WiFi of the train when travelling.  If you are travelling on the train from Paris to Berlin and log onto Facebook to check what your friends are doing, and the person sat next to you is doing the same thing, Facebook will have registered that you two (plus many others) are: connected to the same train company’s WiFi and are travelling on the same journey from Paris to Berlin.

The same applies when you connect to a hotel's or music arena's WiFi.  If you and someone else are connected to the WiFi and accessing Facebook, Facebook will register this and make the suggestion.

The marketing side

If you and your friends like or dislike the same brands, celebrities or articles, then having you friend more people with whom you have similar interests will create an even larger market of people with whom you will share product information, stories and events.

Making friends on Facebook is more than a social game. Facebook's friends suggestions are part of one of the most successful online marketing programs in history. 

In a nutshell

When you sign up for a Facebook account or download the Facebook app, Facebook asks permission to look at your registered e-mail contacts or your phone contacts.  When you grant the site permission (which you have to if you want to use Facebook) it searches your contacts for users already on their database and notifies them of you.

Every profile field you complete you are doubling the source of data for Facebook's algorithm: where you grew up, the hotel you stayed in on holiday, the car you drive, your religion, your favourite drink etc.

Once the Facebook app on your digital device knows the structure of your social network, it will analyse this to suggest not only the people you are most likely to know now but the people you are most likely to know in the future.

In the end, Facebook's “people you may know” system is not magic or malice.  It is really good math.  Suggesting your future friends is not predicting the future; it is modelling the evolution of Facebook's social graph.