I'm not sure it's so much a matter of picking negative consequences out of the blue sky of possible consequences, but rather wondering, "What happens if our software works exactly how we intend and we gain the ability to really manipulate people in a certain way?"

I agree completely that it's hard or impossible to just predict in general how society will react to technology. But in many cases, there are people designing systems specifically for the purpose of getting people to behave a certain way. This implicitly acknowledges that they believe that they can affect behavior in a predictable way. If they can make predictions one way and be correct enough to profit from it, they should be capable of making predictions about things that could go wrong.

I agree completely that predicting something like what Facebook will look like 10 years in the future is impossible, and there's surely a lot of research where the possible consequences are infinite. A lot of results in CS research are like someone developing a new material - "Like, sure, this could be used to make a new, extra deadly tank or something, but it could be used for literally anything." In that sense, it's not useful to make predictions, but I think there's also a lot of research going on at Facebook and Google that looks closer to what I mentioned above, where they absolutely have an intention to change people's behavior and so probably also have the ability to predict the consequences of their complete success.