Facebook didn't experiment with "mood manipulation" any more than OKC. Facebook experimented with changing the site layout and which content and how much to show. Exactly the same stuff as all sites test.
Facebook did theorize that the changes it was making would affect people's moods in a certain way. That might make it sound like intentional mood manipulation.
But OKC's changes also affected people's moods. More than Facebook's, I would guess.
All these sites are constantly experimenting on humans. That's what changing the site content means. Everything that affects us affects our moods and everything else.
I'd also note that the proxy they are using ("apparent mood of subsequent content") hasn't been shown to correlate with the subjects'/users' actual mood.
All the FB experiment actually shows is that by manipulating the mood of content seen, you can affect the mood of content produced.
Here's a couple of contrarian hypotheses: "when some users see more 'happy' content, they feel worse about themselves in comparison, but post more 'happy' content to pretend that isn't so." Or, "when some already-sad users see more 'sad' content, this does not affect their mood directly, but does give them tacit permission to share how they are already feeling. Subsequently, their mood actually improves."
Facebook did theorize that the changes it was making would affect people's moods in a certain way. That might make it sound like intentional mood manipulation.
But OKC's changes also affected people's moods. More than Facebook's, I would guess.
All these sites are constantly experimenting on humans. That's what changing the site content means. Everything that affects us affects our moods and everything else.