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."
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."