I recently added profile pictures to a social network. Unfortunately, the target audience is quite immature and a large majority of the users uploaded NSFW content - mostly pornographic, some anime. Upon closer inspection, a tiny minority of users uploaded child pornography.

Does anyone have any experience with what steps should be taken when this happens? Which authorities should this be reported to? What kind of information should be supplied (server logs?).

For reference, US based startup, with users all around the world. Still figuring out what jurisdiction the particular offenders in this instance lie in.

Thanks