I'm not hatin' on hatin'. I want to be clear about that up front. Condemning rotten behavior is a crucial moral activity, and Facebook is a powerful means of doing so. My friends' Facebook condemnations of sexism and racism and ableism, for example, have increased my awareness of those issues.
And yet... condemning people's bad behavior on Facebook is almost too fun, in a way that niggles at me somehow. Why is it so fun, and what do its pleasures reveal about it?
Clearly part of the fun is that you're on a team. You and your friends get to be on the team of the righteous, aligned together against the target of your condemnatory passions, the person (and more broadly the people like them) who have done that stupid/awful/foolish thing! -- the stupid/awful/foolish thing that you (of course?) would never do. One of the great pleasures in life is building solidarity with like-minded folks in condemning others outside of your group, triumphing over them at least in imagination if not in reality. It's a moral pleasure as well as a social one, and when your condemnation is morally correct and epistemically defensible, it can be entirely good and justified.
Also crucial to the fun, I suspect, is that you receive no genuinely negative feedback for your condemnation. Your Facebook friends are probably like-minded. And if they're not, they're probably quiet. And even if they're not quiet, you can hide their posts or at least, by not "liking" their posts, make their posts less likely to appear at the top of your feed. There's a "like" button, but no "dislike" or "disagree". Now, if you condemn something that is controversial among your friends, you might get some pushback in the comments, but since that's not what we seem usually to want from Facebook condemnations, the activity works most smoothly when we condemn something safe, something we know our friends will also condemn or at least not rise to defend.
Another part of the fun, I think, is a kind of depersonalization of the target of the condemnation. You are condemning a person, yes, but almost always you are condemning a single act, or maybe a few acts of a similar type. The target of condemnation is seen only through one or a few quotations or photographs which might reflect a single moment's poor choice in a complicated life, but which come implicitly, through stasis and repetition, to signify some enduring and central trait in the condemned. You do not see how the person reacts to the condemnation; you do not see the context of the condemned action; you do not see the person attempting to apologize and reform -- or if you do see the person's apology, because the apology is inept (as most attempted apologies and reforms are), it becomes a fresh target for a new round of condemnation, itself again held static and repeated. Maybe one difference between sadism and its lighter cousin schadenfreude is that sadism revels in power over a flesh and blood victim, while schadenfreude (of a certain type) laughs at only a slice of someone, intentionally not gazing upon the target's full humanity. The pleasures of Facebook condemnation are in part schadenfreudist.
I don't think the practice should end. We must laugh at and also more seriously condemn people who do foolish and immoral things; and sharing this laughter and condemnation reinforces community norms. We can't always feel sympathetic pain and embarrassment on behalf of those who go wrong. Yet scrolling down through my Facebook feed that mixes shared indignation and laughter at foolishness in roughly equal proportions with cute kittens and talent shows, I feel that something human is missing -- the perpetrator, as a full person, before and after, with color and nuance and a suite of other traits, sometimes enough to earn forgiveness or forgetting.