So, it can have a massive impact, but it's "just a social network"? How do you know the massive impact is going to be positive, and otherwise it's "just a social network"?
North Korea is also a social network, BTW, and it doesn't give you cancer either, nor does it cause massive floods. Yeah, somewhat unfair way of framing it, isn't it? But then so is bringing up (somewhat) unpreventable illnesses and natural disasters, don't you think? And it's not like social structures weren't responsible for some of the worst things that happened in human history, dwarfing by far any flood.
Would you mind sharing some of the conspiracy theories? Most of the arguments I know are based around the abuse potential of large data collections and surveillance systems and the tendency of some people to abuse power (both of which have plenty of historical examples - after all, that's part of why democracy and the rule of law and separation of powers and all that was invented), but I can't seem to remember any conspiracy theories.
I agree fully that we should take action when something seems plausible and in line with the most accurate, unbiased information out there - I might want to add though that the level of confidence required should be weighted by the expected damage if something goes wrong, the larger the expected damage, the more cautious we should be (and analogously for the expected benefits, of course).
Also, in addition to some paranoid people, there are people who are concerned because of well-informed and well thought out arguments. I for one am very concerned indeed. That does not mean in any way that I can't appreciate some of the benefits (it seems to make electronic communication for the common user very easy, it seems, for example), but I also see large risks, and I think there are alternatives from the technical perspective that should be able to provide much the same benefits without the risks, which is why I think that society should probably try to get rid of Facebook, at least the way it currently functions.