This is a solid example, in relation to the OP comment. Since you didn't know her name. But...there is still a possibility that you had mutual friends and her face popped up. I used FB a lot and FB cycles many friends multiple times, in the hope that someone connects with you. I'm really good a remembering faces and well, this was what happened to me, numerous times. I might not knew the person in January but in July, I'd randomly meet them. FB showed me their photo probably 5 times in the process but it wasn't until the 5th time that I requested.
How is this possible even in principle? Location tracking might be one explanation - they certainly have access to the data - but it seems implausible for Facebook to be suggesting you friend every goddamn person you walk past. They couldn't have known you thought she was attractive, so what gives?
Since anysz mentioned he has 0 friends, there will be no social graph recommendations. Thus less-confident recommendations will show up, including spending some minutes in the same location as someone. It could then consider other factors—opposite genders, relatively close in age, maybe similar interests.
So, since people largely agree on who is attractive, does this mean FB would be recommended this poor lady to all the schlubby loners who wander through her checkout line? "The digital pimp, hard at work."
As long as there is nothing forcing people into it, I think it could be great.
We don't know what we don't know to paraphrase that famous romantic Donald Rumsfeld. Who knows if the person behind you on the rollercoaster or grabbing a taco might be the yin to your yang.
I just wish Waze could tell me who my ideal carpool partners might be.
You probably don't use your real name for the FB account, do you? (Ruling out the "she saw your name on your moneycard and googled you on Facebook" hypothesis)
