Many people across the US are unable to access the Facebook website right now. We've confirmed outages in both San Francisco and New York City on PC and mobile browsers, and Downrightnow.com lists the site as having experienced "likely service disruption" for over two hours. However, the social network remains accessible via mobile apps, and we've found that typing the URL as "www.facebook.com" has helped some users get online. We haven't had any trouble accessing the site from outside the US, either.
We've reached out to Facebook for more information on the issue and will update you when we hear back.
Update: Facebook says the problem should be fixed by now, and has provided us with the following statement:
"Earlier today we experienced a DNS issue that briefly prevented people who typed 'facebook.com' into their browsers from reaching the site. People who accessed the site using a mobile app, typed 'www.facebook.com' into their browsers, or used a bookmark or a search engine to navigate to the site were not affected. We resolved the issue quickly, and the fix should be fully propagated now. We apologize for any inconvenience."