Have a look at this Read Write Web article, specifically the paragraph in bold and the comments.

Have a wry chuckle, or maybe weep for the future of humanity - your call. Then pause, and worry about information architecture.

The short story: Read Write Web bumps up the Google rankings for "Facebook login" at the same time as Facebook makes UI changes, and a few hundred users get confused and leave comments on Read Write Web complaining about not being able to log in to their Facebook accounts.*

Blindly clicking the first Google result is not a navigation behaviour I'd anticipated for folks visiting big names sites like Facebook. But then, I use Launchy and don't know where any of my files are, depend on Firefox auto-complete, view Facebook through my IM client, and don't need a map to find my backside with both hands.

Not all our users behave in the same way, which means not all of our architecture is within our control, and people can get to your content in all sorts of ways.

Even if the Read Write Web episode is a prank of some kind (there are, after all, plenty of folks who enjoy orchestrated trolling) it's still a useful reminder. Your users may take paths through and to your content you cannot control, and they are unlikely to deconstruct their assumptions along the way.

I guess the meaningful question is: can you still support those users? If they get to you from Google instead of your front door, does what they find still make sense? Does your information architecture still work if your guests come in through the bathroom window?

Ok, so here they broke into the house next door - you can't be expected to deal with that. But the rest is well worth thinking about.

Other off-site interaction

It's rarely going to be as funny as the comments at Read Write Web, but your users are going to do, say, and read things they think of as being about you and your products, in places you don't control.

That's good. If you pay attention to it, you get data. Your users get a better experience. There are easy wins, too.

Blogs, forums, social media &c.

People may look for and find help with your product on blogs and forums, on Twitter, and what have you. They may learn about your brand in the same way.

That's fine, it's an interaction you can be part of. It's time-consuming, certainly, but you have the option. You won't get a blogger to incorporate your site navigation just in case your users end up there, but you can be there when they do. Again, Anne Gentle, Gordon McLean and others have covered this in more depth than I could.

Direct contact

Sales people, customer care, support, they all talk to people. Are they sending links to your content? if so, which bits? Do they know about all of it? Do they have the content they need to support them - messaging that funnels sales, FAQ that are realistically frequent, detailed examples of things people want to do, that kind of thing.

Are they sending links because users can't find the good stuff? Are they sending précis of your content, or re-writes, or brand new stuff? If so, does that mean your content isn't up to scratch, or that you've got content missing?

Direct sales/care/support interactions are enormously valuable, and can help you know what content your users find useful.

You can't have a table of contents or a "See also" in a phonecall, but your content strategy can support more interactions than browsing.

*Passing observation about Facebook. For plenty if folks, it is  the internet. Its services are simple versions of what a lot of people use the internet for, and they're aggregated into one stop. Flickr, Vimeo, Wordpress, Twitter, LinkedIn, and all sorts of games, have Facebook doppelgangers that are not only friendlier to entry-level users, they're right there, behind only one layer of authentication. As such, it could own a lot of interaction convention. Heavy users may well not be tech-savvy, and be quite change averse. That doesn't make this episode not dumb, but I'm happy to go easy on 'em.