Too lazy to google, but I seem to recall something in the last months about a similar thing, where people were offered a bar of chocolate or something in exchange for their password.
First, it was over 2 years ago.
Second it was apparently 20% of people gave their passwords in exchange for chocolate.
However, the key thing is - the survey had absolutely zero way of confirming whether the passwords were genuine or not.
You know what? Some random in the street offers me a bar of chocolate in exchange for my password, I'll gladly trade; I end up with a free bar of chocolate, they end up with a garbage string of characters which isn't my password to anything at all. Seems I would be included in that 20%, but my security would have remained uncompromised and I'd be better off to the tune of 1 bar of chocolate.
which all just goes to show that the survey was crap, the results equally so.