A common conversation on Facebook (-like) site

I get a mail from Facebook:
"Boglarka Vasilovici added you as a friend on Facebook..."

I have no idea who she is.. i check her profile .. we have 1 friend in common.
just in case I sent her a message..

"- Do I know you?
 - It's unlikely...
 - .. And you know me? or why do you added me?
 - 'coz we have a friend in common
blablabla "

I dont get it .. I mean of course I have "friends" who I dont know personally or talked only a few times.. but this is far from everything