On Facebook and Twitter I've never seen a friend or someone I follow make some disgusting comments like I've seen on Yik Yak, specially the top, upvoted stuff. I never used Tumblr (only when visiting a blog) and only visit Reddit sporadically. So no, I can't say the same.
Try running a search on Twitter--you'll see some nasty stuff, depending on the search term.
Yik Yak is basically just location-based search, so there's no social filtering. (As opposed to on your Twitter feed, where you create a social filter by selecting the accounts you follow).
