If you look at social science as an entire discipline (separate from "hard science"- which sounds dismissive- "Hard science" is able to achieve better precision because its experimental subjects are easier to control! in those ways it is easier!), it is so fraught with confounding factors and difficulties applying controls that eventually you have to choose a lower standard of evidence than "perfectly rigorous". One standard of evidence, or maybe better stated as epistemological standard, is the precautionary principle. If cell phones are an unknown threat, but a "creepy" one, where there certainly seems to be enough evidence that one should treat them with caution- treat them with caution. Especially when you can tell that the benefits of certain behavior patterns don't outweigh the risks.
There are lots of areas of our lives where we lean on the precautionary principle WITHOUT needing bulletproof justification.
Thing is, we already applied precautionnary principles in a very organic fashion: hight end phones have always been expensive, and very few kids got access to phones able to access messages, email, then social networks.
Be it Blackberry or Vodafone/docomo phones it took years before regular teenage kids could buy them in significant numbers. Same for the iPhone, it's not like millions of teens got their hand instantaneously on a 3G the minute it went out.
By the time it became widspread enough, we already had devices in the wild for a decade or so (for reference imode launched in 1999).
We might want to revise how smartphones are regulated and/or social media policies, but IMHO we're far away from the simple "we don't know let's be cautious" phase, and there is a burden of proof on what exactly needs to be regulated to what expected effect.
If it's a precautionary principle, it should be called so. One should say "We don't know if smartphone is the cause, but by precaution, ...", which is clearly not the conclusion provided here.
You cannot say: I don't know if X is harmful, but by precautionary principle, X should be kept an eye on, so I can say unscientific conclusions such as "X is harmful" and everyone who complain that it is scientifically poor are incorrect.
Also, there is also a risk i precautionary principle: hiding the real cause while hurting the "good usage".
There are lots of areas of our lives where we lean on the precautionary principle WITHOUT needing bulletproof justification.