I am not surprised at all. My NON-ADULT yahoo group was spammed so much with porn, I had to begin moderating posts. I got tired of it and shut the group down.
Much of the porn was lolita and some of the girls looked to be pre-teen. Most of it came from ips in France and Russia. Yahoo did nothing. Occasionally when I drop by yahoo groups, I see that some of the spammers are still posting.