One of the most telling moments in the spectacle of Mark Zuckerberg’s congressional testimony last week was when Representative Billy Long, a Missouri Republican, warned the co-founder of Facebook of what Congress was likely to do about its multiple concerns surrounding the social network. “Congress is good at two things: doing nothing, and overreacting,” he said. After years of the former, he said, “we’re getting ready to overreact.”

That sounds worrisome. But I sympathize. Doing nothing and overreacting make sense when you have no clue of what is going on. And we don’t. The cloud of questions aimed at Mr. Zuckerberg — Is Facebook too dominant? Does it censor information? Whom does it share our data with? Does it help sell OxyContin? — suggests that we don’t really know what the problem with Facebook is. It also suggests we don’t understand what Facebook does.

That goes for the entire data-driven ecosystem, from Google to the auto companies riddling your car with sensors that can tell where you’ve been and how fast you got there. And that puts policymakers in an uncomfortable spot.

The crucial issue for Congress, government regulators, members of the public and even Mr. Zuckerberg is how much all this data-driven stuff is worth to us. What do we stand to lose by, say, sharply limiting the data these companies can collect? What do we stand to gain?