how did a friend on my facebook hack me so quickly?

[Sarzzzz]

I had an argument with this guy ive chatted to for ages on FB. He told me to not wind him up otherwise he'd hack my account - anyway, i didnt listen and within 5 minutes he'd got my password and sent me a message to prove it.

I won't report him because we are normally good friends... but he wont tell me how he did it, all he keeps saying is 'never underestimate anyone;

does anyone here know how he's done it? thanks... it was so very quick and its bugging me lol