Most groups that would be on such networks (other then child porn producers/consumers) would be mostly political in nature. As such their use of said network would mostly be to distribute textual material (forums/blogs/manifestos/howtos/newsfeeds/ebooks/etc). It would be material that takes up less space, but is actually more time-consuming to produce. For example, writing an ebook takes longer than rolling the video camera for a couple of hours, and produces a much smaller (space-wise) amount of content.
Not really. Astronomy generates nearly endless amounts of data (although they are no longer likely targets of haressment) and the other groups would be posting plenty of video (mostly recordings of police stops, howtos, etc).
And while 16 year old sexting would be considered cp I highly doubt they would be uploading it to any such networ, if only because they don't want others to see them.