People loathe having multiple credentials, so if they can manage their own pictures on Flickr, and at the same time be a part of the 'Iams Dog Lovers' group to share stories and photos (I made that up), then why wouldn't they? For the brand, it's less budget, less development, simpler management, easier promotion, greater traction, bigger audience, etc. It just makes sense.
Facebook Fan pages are the obvious answer to brands looking to aggregate their fans in one place. Content management, comment moderation, no development, no cost (clarify: no maintenance cost). Plus, everyone is already there.
What about if your brand is something very niche-- should you use Ning, or another platform to make an entire social network just for your niche group? Afterall, naked cyclists (example) don't use Facebook, right? At the end of the day, pretty much anything you can think of, has an audience, and is represented on a major network. Whether they've been branded and aggregated--that's your job, but the audience is there.
What if you're Monsanto and you do great business with farmers (who are essentially end-users/consumers) and ag retailers all over the world, but a few pesky operations issues have generated some negative PR. You may want your own community to talk about very specific industry things, possibly content that you don't want indexed by search, but that isn't necessarily secret, and of course, the ability to heavily moderate and track conversations. You may not want to discuss a new Soy variety on Facebook. A company like Monsanto may want their own community.
So where do we draw the line? It think it's audience. If you speak directly to consumers, you may as well be where the consumers are. Nike, Dell, Air Miles, Shoppers Drug Mart, Canadian Tire. If on the other hand, your company offers a service or product to businesses, then even though those same people are on Facebook, they, for work, may be the audience for a private community, or at the very least, a stand alone forum or discussion board. Monsanto, IBM, SAP. I draw the line at B2C vs. B2B.
There is however a middle ground that I would like to call the expertise sector. Think of a key audience group like Microsoft .NET Developers. They could (and probably do) have a Facebook page where they can get news, links to resources, etc. But an expertise group like developers need a forum for discussion that would really bog down a Facebook Fan Page. They need sub-topics, and moderators with expertise, and the ability to share more than just links, but maybe code or files--- this is all beyond what Facebook can really do. So they're not B2B, but they are an audience that needs their own community.
When you're evaluating whether or not your brand or company needs to come up with a social network of some kind for your key audience, ask yourself this: Can we use an existing platform? If yes, then the likely answer is that your audience WANTS you to use an existing platform.