Did you add each others' numbers on WhatsApp?
Or if you moved due to work and you got added to a company-wide address book which is also synced to employees' phones and in turn got synced to WhatsApp, Facebook's systems could probably match up names to suggest a connection.
Although, why only these 2 people and not others from the company? Scarily there's probably enough data between you 3 for FB to know that you would be compatible with each other, so maybe Facebook figured the same thing you 3 figured out in real life -- you hung out with each other so I would deduce you found these 2 co-workers interesting. But I doubt anyone got paid to make such sophisticated stuff, there's probably a simpler logic that offers a quantity but not quality of "friends".
I still want to write a short story that starts with a guy saying "OK Google, get me a girlfriend", and an algorithm that slowly pops up suggesting events and locations for him to visit, knowing that a girl that he'd be compatible with will be at the events/locations...