Facebook security a joke
I hate Facebook, but run a community page so I have to maintain an account on the site.
At least every second time I login into Facebook, I see obvious scam postings supposedly made by friends (ie they've accepted a dodgy app that posts on their behalf). I find my time is split equally on Facebook into managing the fan page, and telling friends they've falling for a scam.
The worst thing, is that even though they listen and realise it is a scam, a couple of weeks later the same people for the latest scam app doing the rounds on Facebook.
One friend had accepted an app which posted on their wall every hour or so, and on the walls of their friends. You couldn't delete the app from your profile and a few friends had fell for it. It took Facebook several months before they addressed the issue, even though it was reported to Fb repeatedly, provided screenshots of the app, providing technical details, etc. Facebook did not respond to anyone and allowed it too happen.
I personally think they ignore all the scams, until a scam has reached a threshhold of complaints, it could be a 1000 complaints, 10,000, 100,000 etc. Who knows, Facebook will never tell.