I think web-site's are the same way. You build a site intending for it to solve one problem, but once users get their hands on it, you find out its good for something else. The question is do you embrace that usage or shut it down. Here's a couple of examples:
* Craig's list - sure everybody's favorite lovable free classified site is a great way to get a job, find an apartment or join a band. Funnily enough, it turns out most people use it for the "erotic services" section. Intentional? Who knows. But the edgy content and the SEO-able nature of that section has proven an effective way for them to boot up local cities. Think the local newspapers websites in these towns would follow this strategy? Ha! And you know how well their sites are doing....
* Google - sure organizing the world's information and making it accessible is a lofty goal, but in reality, lots of the information being accessed is not so lofty. According to these stats, over 7% of the search query stream is actually porn. 7% may not sound like a lot but it's the 4th highest category and the top category is "other." Gee, I wonder why they have image and video search...?
* Facebook - it turns out that despite all the talk about the social graph, Mark Zuckerberg, etc., one of Facebook's biggest draws is that it's a photo sharing site. The killer app is that bit where you tag the individuals in each picture. The people I know who spend hours on that site spend most of that time looking at tagged photos.
* Pownce - yesterday's big news was that would be Twitter competitor Pownce was shutting down. Valleywag had this little nugget in its write-up: "Pownce allowed users to send each other short messages and, most importantly, share files; bootlegging MP3s was a popular if unacknowledged use."
None of these sites made these particular usages part of their core messaging, but they otherwise have have embraced them. Obviously, Pownce didn't end up succeeding - but that's probably for a host of reasons. Maybe they should've promoted the file-sharing bit more....couldn't have turned out worse.
As for the others, they certainly have done well. Probably one of the reasons is that they listened to their users. And by listen, I mean watch their usage.
Anyway, back to Ning. So your customers want to use your service as a place to create adult social networks? That use doesn't work for you? Fine. That may be the right decision for them. But as Chris notes, it's probably not the best way to treat your customers. And any company should be really careful cutting off legal uses of a product, just because they're not what was intended.