I think pretty much any network with social potential will be used for porn spam eventually. (for example, I've noticed a bunch of Google Hangouts groups I've been added to for that purpose)
First, you're conflating child porn with regular porn.
Second, certain services are more often used by serious deviants, and it's a huge reputational, moral, and legal risk. If you're running a business, or a user of that service, you can't afford to bury your head in the sand.
My apologies if you read my message as a dismissal; I wasn't trying to communicate that message. My point was that all social networks are targeted for abuse (if you don't see it on your favorite network, it's because that network probably puts a lot of time and effort and money into fighting it, not because they aren't being targeted)
