These are some of the answer I got for the question in Facebook’s own group and they sound like FB's spying on you and recommending friends.
Answer by a facebook user :
Maybe it cross-references GPS/location data (but that doesn't really make sense for folks that haven't been here in forever) or maybe they have my phone numbers in contact lists, etc? I think it's pretty creepy.
Answer by a facebook user :
Facebook is definitely recommending people who search you. There are people who have no mutual friends with me and I have never contacted by phone or email, and Facebook recommended I add them as friends. I have met them in real life, but not college or work and they don't live in my city.
Answer by a facebook user :
People you search for- this history can be deleted, it is part of your activity log
People who search for you
Being tagged in a friend's photo or album with other people in it
Large number of mutual friends
Network affiliation (school, work, city)
Group affiliation
Attending the same event
Friends of connected social media accounts
Associated email addresses and phone numbers from your mobile (only if you use the Facebook mobile app and allow access to your contacts)
Your email addresses or phone numbers are saved in someone else's mobile AND you associated your phone number and email addresses with your profile
Friends of people you have blocked
Opting to upload your email contacts by temporarily providing FaceBook your login and password to an email account.
Point is, there are countless ways that FaceBook can recommend friends. Their business model is to have people connected so they can sell market research, demographic data, and cater to you as an individual instead of as part of a large body of people. So it makes sense they will do anything they can to build people's connections. Not saying what's right or wrong, just saying what it is. FaceBook does not purposely intend to injure anybody with their recommendations. It's also within your power to see only what you want.
Answer by a facebook user :
If you are really concerned about your privacy, I strongly urge any readers of this thread to check your privacy settings and seriously adjust them to your liking. Changing your visibility settings to only those that know your affiliated email account might help with unwanted friend recommendations. No sense calling out privacy concerns if you're not going to do anything about it for yourself. Delete your search history if you have to. Don't put up with a stalker, block him/her already. Unsubscribe from News Feed spammers. Sort connections into close friends, family, acquaintances, and those with restricted privileges to your profile. Take charge of what you see by adjusting your privacy settings instead of endlessly scrolling through the News Feed.
Answer by a facebook user :
From what I can see, Facebook will access anything on the device you're using (PC, laptop, mobile phone) that might contain somebody's name, and then suggest that person as "people you may know". For example, I use Google calendar for my business appointments. Many of the people suggested to me by FB as "people you may know" are clients with whom I have recently made appointments on Google calendar. I have never searched for them, nor they for me. As far as I can tell, these recommendations are not just based on "searches", but also on the contents of other programs and applications on your device . . . any place a name can be recognised by Facebook's algorithms.
Answer by a facebook user :
I think FB has access to your yahoo or hotmail contacts and recommends friends based on that because I've had my landlady pop up on mine. We have no mutal friends and I never met her before I rented the flat for her
A forbes finding from another user :
One area user told me, “this morning Facebook suggested my scary ex out of nowhere. There’s no contact information in my phone or shared friends with said entity.” I pressed for any commonalities and there were none, not with email, not with the previous profile — nothing. It’s unsettling to imagine that this is happening with many users, some maybe faced with something worse than just ‘scary.’ That same area user then had a very profound thought, “what if Facebook is suggesting people that are actively searching for you — whether they be long lost relatives or straight up stalkers?”
Facbook Statement shows it want to know every human and connect them on their own liking and choices to benefit the system with more robust add spread and earning from the same.
“Facebook just bought the web’s most talented and creative scrapers that have gotten around everyones rate limits and detection systems.” Said another person we spoke with this evening who is knowledgeable of Octazen’s product, “Facebook is so sanctimonious about protecting their own user data through Facebook Connect, but Octazen has been scraping user data for years off terms of service and then reselling it.” It was in 2010 and it started the battle for facebook to cover the earth in one ambit of social network and that is facebook the reason they proceeded to buy 50 companies in total spending of $22 Billion. Now that you know how much money.
A nice infographic showing facebook journey with other ventures :