Tough to say, but if this link is right: http://video.stackexchange.com/questions/8850/typical-size-o...

Then being conservative, we can say an hour of video is 1-2GB of data, so between 500 and 1,000 GB of data uploaded (just to YT) every minute.

Accounting for mirroring and compression, 525,949 minutes per year * 1 TB per minute = ~526 PB per year

Ironically this would be about 65,743 8TB HDDs, which is close to the number in this article :)

They also store several copies of each video (different codecs and resolutions).

I had heard 1PB/day somewhere...