Replying to What's with the porn in my newsfeed..?

So, today I log into my Facebook and BAM, 13 year old Indian girl tits. Then my sister logs in and WHAM, gay sex. Later tonight I log in and BLAM, Justin Bieber sucking a massive Mexican cock.

Why are these things showing up in my supposedly PG-13 newsfeed? I know it's because a friend of mine commented on the picture, but how do these pictures get on facebook with thousands of likes/comments?

Has anyone else noticed this?