p.s. if a random person searches for me on FB, then I see him as a friend suggestion?
In case of random persons, there has to be something common between you two... such as interests, schooling, likes, geo-location, mutual friends etc. But if a random person visits your profile several times then Facebook sees that as an "interest" and asks you "if you know that person."
I closed my FB account in 2007 and I have never used/installed WA in my life. Nobody knows my personal mobile number outside of my immediate family members. It's on full DND ever since this service was offered. And yet my brother sees my long-lost friends in his FB suggestion list. There is absolutely nothing common among my friends and my brother. My brother doesn't use WA. And I have never met/spoken to those friends in a decade. So, they don't know my phone number either.
My brother has a not-so-common name and my friends know that. When my friends couldn't find me on FB, they would have searched for my brother and checked out his profile in the hope that they would locate me on my brother's friend/family contact list. For my brother, they were totally random people and yet FB thought they might knew each other.
But that's how people used to discover their secret admirers during Orkut/HI5 days

. These social networking sites would let you see who visited your profile, when and how many times. There were lots of other sites that used to collect such data and do a matchmaking on the basis of interests. In fact, Facebook too had this profile-visitor feature before people started to cry privacy.
Kinda moot anyway, anyone using whatsapp/facebook/truecaller has already said goodbye to privacy...
Yes. If you use internet, there's no point losing your sleep over privacy leak.
Funny thing: For a long time my family couldn't find out how I was able to guess their surprise weekend dishes with so much accuracy. We all use different devices but share the same internet connection at home. By every Thursday/Friday, my Youtube account would start showing me list of recipes under "recommend for you" section. I have a Youtube account exclusively meant for downloading on PC and it's not shared with anybody. But thanks to omnipresent Google, my family members' secret Youtube recipes are no longer surprises!