Now that 500 million pairs of eyes are potentially on any brand’s Facebook page, spammers are stepping up their efforts to secure a piece of the viewing time, and they are already posting porn links and other nuisances on the biggest brands’ walls. Why exactly are these pages so vulnerable, and what are brands and Facebook doing to keep the horny-gal-wants-you posts to a minimum?

To be sure, spammers have been targeting the most popular pages on Facebook for some time now. Last month, we reported on Dr Pepper UK’s social media mishap when it posted some very inappropriate material on its fans’ profiles in a classic example of a social campaign gone wrong. The core of Dr Pepper’s mistake rested in the fact that it made reference to the viral porn video “Two Girls One Cup”, which didn’t sit well with the mommy bloggers, or, moms. Just as egregious though was all the porn spam visible (and still today) on the Dr Pepper US page. Doesn’t anybody at the softdrink brand care about the hardcore messages posted their daily?

At the time it seemed like a contained event of porn spam, but AllFacebook.com reported yesterday that porn spamming has hit hard on a wide variety of brand pages, including celebrity pages like Michael Jackson and Britney Spears. Even the Rolls Royce page has suffered porn pains.

It’s a classic case of sloppy community management. There are plenty of examples of brands simply abandoning old marketing campaign Facebook pages, as we’ve reported here. But this is a new form of neglect. Put simply, failing to keep a close eye on Facebook wall conversation and failing to keep discussion relevant can be extremely dangerous for online brand presence.

Of course, many brands are doing it right, understanding full well that the dialogue on the brand wall is a reflection of what that brand stands for. Back in June, Starbucks set a precedent for muzzling unrelated conversation on its fan page when a political activist spammed the fan page with anti-Israel messages. On a more general note, the Lady Gaga PR team has proven to be extremely attentive about managing its Facebook page and other social channels. Not surprisingly, the Starbucks and Lady Gaga Facebook pages (two of the biggest on the network with 13 million and 16 million fans respectively), are two places where we have been unable to find any trace of malignant spam.