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YouTube.com itself appears to be working, but links to individual videos are extremely sporadic. Downrightnow.com has called a "likely service interruption" since roughly 4 p.m. ET. Pingdom.com shows several outages over the course of the previous hour. This is a shame, as the Google-owned video service had an unblemished 100% uptime record since the middle of May, according to Pingdom.
We've reached out to YouTube and will keep you updated on the situation. In the meantime, let us know in the comments if YouTube has stopped working for you -- and what you're doing instead. Did business productivity just go through the roof?