You don't think they monitor YouTube for terrorist video?
You don't think they monitor other video sharing sites for video of terrorist organisations releasing propaganda? That seems like useful intelligence, and they'd be foolish to ignore it.
Suppose a 10MB youtube video is watched 100x. That's 1GB traffic. The NSA could record the video (10MB), then simply record who watched it (maybe a few dozen MB in http headers, etc -- call it 40MB)
They could report they only monitored 50MB of 1,000 MB, or 5%.
If the video was watched 1,000x, they're "monitoring" around .5%. 10k views, they're monitoring .05%.
Pretty easy to see how they can report a true number that also doesn't reveal the extent of their wiretapping.