Sure, the referrer can be spoofed if you can set arbitrary headers, but you can't set headers on iframe requests anyway (and even XHR explicitly disallows setting Referer)
Also I suspect this service is fairly self-regulating. Facebook users are generally careful about what they broadcast. The author gives the captcha trick used by porn sites as an example...how many people are going to broadcast their taste in porn?
Seriously? Maybe among your tech-savvy friends, but the majority of Facebook users have no idea what they're doing when they type something into the box and click "Share."
A few minutes over at http://failbook.com/ is enough to point that out, and those are just the egregiously bad / hilarious cases.
there are already lots of spam websites and fb apps, that trick into being a fan... i mean, "like" pages using js. this iframe only makes it easier.
i can even imagine spam js links altering a legit iframe, hoping a user clicks it afterwards.
Not a big issue now, but if lots of pages start using this capability, it could become a problem, albeit of a very minor variety.
not a terrible confusion or potentially too sinister, but a bit more attention than usual is required than the simple share.
Trend?