Update: 6:30 P.M. Pacific: Users on Reddit have reported that attempting to access Facebook with the "www" prefix resolves the problem, while the "facebook.com" root domain continues to experience issues. Reports around the web also suggest that Chrome users are disproportionately affected by the outage.
We've grown to know Twitter's fail whale pretty well at this point - and Tumblr seems down half as often as it's up... but Facebook? Facebook doesn't just go down - not often, anyhow.
As of 5:15 p.m. Pacific time or so, Facebook isn't loading for some users. We've confirmed the outage with Downrightnow and a handful of other monitoring sites, which lists the social network's status as experiencing a "likely service disruption".
It remains unclear at this point if the Facebook outage is the result of an internal issue, like the DNS misrouting blip back in October 2012, or if it's an attack of some kind (our money is on the former).
Here in Portland, Oregon we're experiencing the service disruption, but anecdotal reports suggest that many of our contacts in San Francisco are unaffected at the moment. We've reached out to Facebook about the outage, but in the mean time, you'll have to poke and like in the real world. That or tweet, pin, +1 and tumble for a little bit.
Is Facebook down for you? Let us know where you are and what you're seeing.