Promoted Trends, the sponsored slot that appears at the top of the trending topics list on your Twitter page, are important to Twitter's financial future. At the reported cost of $120,000 a day, it's a valuable, in-demand piece of advertising real estate, putting brands front-and-center on one of the hottest media properties around. But premium as it may be, the promoted tweet is often surrounded by an unruliness that makes it not unlike a high-end restaurant with an absolutely filthy bathroom wall.

Last weekend, Discovery Channel saw this when it bought the slot for its Shark Week programming. Yet on Sunday, there was a point when the most popular organically-occurring trend was the distasteful #reasonstobeatyourgirlfriend. To be sure, a good chunk of the Shark Week tweets were positive, but more than a few yoked together the two hashtags, dragging the venerable paean to aquatic predators into some murky trenches. Example: "She won't shut up during #sharkweek!!! #Reasonstobeatyourgirlfriend."

This sort of thing happens on a regular basis. A rather garish example occurred back on April 20, when Lexus CT Hybrid held down the promoted spot. For a good part of the day, it found itself adjacent to Adolf Hitler (it was his birthday) and Columbine (the massacre's anniversary). And to celebrate 4/20, a big day in cannabis culture, there were some marijuana-related hashtags like #uknowurhigh.

Asked to comment, Brian Bolain, Lexus' national manager of marketing communications said: "The very nature of social media is real-time conversation. We will continue to work with our partners on an ongoing basis to test , learn and develop best practices and guidelines in this space. That said, we also feel social-media users can distinguish between trending news topics and promoted topics. It's not that much different than if we purchase an ad in a newspaper that is reporting about the day's news, good and bad."

The difference of course is that newspapers don't publish articles that crowdsource justifications for thrashing your partner or lists of songs about rape.

And that kind of cuts to what's interesting about the Promoted Trend vehicle and, more broadly, social media's redefinition of "news" to something like whatever is popular at a given moment. There's a very long-held concern about context in advertising that doesn't seem to apply to Twitter -- even though that concern hasn't dissipated in a digital age that 's seen media plans become massively complex and less human-driven, with digital ads often served up through automated processes rather than direct media buys. When Wonkette ran a post making fun of Trig Palin, Sarah's child who was born with Down Syndrome, brands from Papa John's and Huggies to Vanguard bailed quickly, prodded along by a very vocal boycott from conservative bloggers and news organizations. There's enough concern about avoiding these unfortunate adjacencies to support a handful of businesses whose central job is to make sure that their clients ads aren't placed next to nasty stuff.

So what makes Twitter special? Why do brands put up with being next to nasty hashtags? Well, Twitter and Facebook are sort of proving grounds for marketers who want to show that they understand how to forge conversations with their customers. Whether a paid slot like the Promoted Tweet is any more conversational than a 30-second spot interrupting your view of say, a tiger shark feeding frenzy, is a matter for debate. And the fact is , having a strategy for Twitter is more important than any run-of -the-mill banner ad placement that might net you a few cheaply-acquired eyeballs and little else. The platform enables water-cooler conversation being conducted by millions over some period of time and the clearest distillation of that is the trending topics list.

But with power comes responsibility. Said David Armano, exec VP-global innovation and integration at Edelman Digital, unfortunate placements "will probably not stop advertisers who want turnkey solutions from using the service, but it does underscore the need for not only Twitter to be monitoring trends, but brands to be monitoring their own activities (such as social media placements) so they potentially head off a situation before it escalates. "

Last week, Twitter signaled that stricter moderation is coming. In response to some complaints about #reasonstobeatyourgirllfriend, CEO Dick Costolo tweeted that the trends are chosen algorithmically, with Twitter editing out ones with obscenities. He also indicated he'd like to see the "clearly offensive" eliminated as well. Mr. Costolo's Tweet was in response to one from Candace Kuss, a PR person who was annoyed that the girlfriend-beating hashtag made it through while #fuckyouwashington, blogger Jeff Jarvis' bilious reaction to the debt ceiling quagmire, didn't. (Even though being edited off the list didn't stop that hashtag from being retweeted far and wide.)

I asked Ms. Kuss, London-based director of planning-interactive creative strategy for Hill & Knowlton EMEA, for her take on the issue as a marketer. "As a brand steward for our clients, nasty and offensive organic trends don't add to the appeal of the Promoted Trends product, but I am not sure if that would totally prohibit an ad buy, especially given the ephemeral nature of the trend list. But if I was advising Twitter, I would be concerned. While personal users might prefer organic trends to freely reflect pure volume, professionally I think it would be good to see Twitter extend their ban of the obscene to include the dangerously offensive and to publish a clear guide to their curation principles."

There remains a big question: How will Twitter differentiate the meaningfully obscene from the pointlessly offensive? "It's a tough call," said David Berkowitz, senior director of emerging media and innovation at 360i. "But there should be some moderation. You want the platform to remain in the hands of the people. There are people who use Twitter in great, great ways but there are also a lot of jerks out there."

Its success in tiptoeing through this minefield will determine in part whether Twitter can retain its vital role as a mirror to culture, reflecting some of its rawest forms, while keeping a tidy enough home so that advertisers visit. Only time will tell if that works, but in any event it's sure to be a rather bumpy ride for brands that go along for the ride.