I don't think the title quite follows from the article. There might be an association, but even supposing there is causation the independent/dependent variables aren't necessarily known here: people with low social trust (for other reasons) could be more likely to engage in online social networks as opposed to meatspace social networks.
[...] In particular, while the effect on well-being has been largely explored in the case of face-to-face interactions, we do not have conclusive evidence about the endogenous relationship between online interactions and well-being. This weakness suggests caution about the generality of the results provided by previous literature. Individual effects such as personal characteristics may be correlated with both participation in SNSs and well-being. Happier people may also be more outgoing and open-minded, and may have a higher propensity for various kinds of social interaction. [...]
It's fine to be skeptical, but couldn't you at least do the authors the courtesy of looking at their methods and pointing out where you think they fall short?
However, you are overstating the case for the author's having dealt with reverse causality. From the conclusion:
When we addressed causality in IV estimates, the significance of the correlation between participation in social networking sites and subjective well-being disappeared.
See my top level comment for my own take on the article.
My gripe is with reflexive and unjustified statistical one-upmanship in HN discussions.