How is this possible?

Yesterday I got an email that someone had added me as a friend on Facebook – which I suppose is what it is, except: I’m not on Facebook!
How can someone add me when I’m not registered? And yes, I did double-check that I don’t have an account I forgot about or something…...and I don’t!
Is it just me or is this a severe flaw in their set-up – I think it certainly confirms I was right not to join it in the first place.

What’s your take on it? Has this happened to you?