If you know or have good reason to beleive that a crime has been committed then you have a duty as a citizen to report it to the proper authorities. If it were a bit of inappropriate swearing or nudity or something then I would think Facebook would be the correct body to report it. If you think there's s crime, police, in my opinion, although you don't say what the cruelty is, I'm assuming some sort of dog fighting or something, Rather than a dog that looks a bit tired or muddy or similar?
I once clicked a link to a video of a girl laughing as she drowned a litter of puppies in a river, it's haunted me.