It seems far more plausible people are picking up behavioral norms than being emotionally influenced. If none of your friends ever complain on facebook, are you going to be the one "grump"? I didnt read the study admittedly but this feels off. I thought it was axiomatic by now that what people post to facebook is 90% about how people want to be percieved, not who they really are.
Sure, the reason why people replicate each other's sentiments is often to be normative. The interesting part about publicly expressed sentiments though is that they tend to be subconsciously adopted by those expressing them even in the case of initial insincerity. This is essentially Cialdini's consistency principle.
I think that heuristic attributes more foresight to people than they actually exercise in practice. It's more like the impressions one anticipates making on others create incentives and disincentives that shape behavior...but this applies offline as well. Even IRL, everyone's behavior tends to be bounded by "how people want to be perceived, not who they really are."
