Now, Parks and Rec won't necessarily act immediately. In fact, they've admitted that they do only what they can. But they still act if possible. Last month's crackdown on Golden Gate Park was partly spurred by complaints. And smaller things like graffiti or an over abundance of dog sh-- are often acted upon.
If you want to see the types of complaints posted as well as for which park, check out the story in the Examiner about the site.