Although I'm a bloke and have watched porn, there's nothing remotely appealing about this and can fully understand the concerns expressed.
I would in the past have slated the IWF (Internet Watch Foundation) as sometimes being overzealous, but in this case, while they generally deal with child porn and abuse of children, there is one entry on their reporting web page which seems to fit the bill, namely
Inappropriate chat or behaviour with or towards a child online.
In the case of this promotion, tempting teenagers to allow embarrassing status messages to be displayed, with a chance to win 1000 pounds, there are two areas where this is "inappropriate" - first in that the examples on the website don't go anywhere close to what is actually being put as status messages, so a young person is being misled about how "embarrassing" the messages might be - and second that the "audience" will likely be other teenagers, who will be seeing the status messages of others.
The IWF has somewhat draconian powers, placing any of those websites (including Facebook.com where some status messages may be seen) could lead to all UK ISPs blocking access.
Perhaps it is worth using as a threat, that closing this promotion seems the best option (and if not, then the 'further action' might lead to blocking of all Coca Cola websites, the PR firms which mention this promotion/ encourage any teenager to sign up, and finally FaceBook.com which would be major shock / news if blocked for even a couple of days.
The "PR" story would be that of a massive black thundercloud over Coca Cola and any agency they had used, and would go worldwide. I think the threat of reporting CC to the IWF might shake up a few senior staff (though others might be complacent if they only think 'nothing to do with child porn' and shrug their shoulders).
Sorry Facebook users, if it was reported, I think IWF could blacklist FB until such time as the story was out about a scandalous promotion run by (an agency for) Coca Cola, and block could be kept until promotion cancelled.