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The rise of social media has provoked both optimism about potential societal benefits and concern about harms such as addiction, depression, and political polarization. We present a randomized evaluation of the welfare effects of Facebook, focusing on US users in the runup to the 2018 midterm election. We measured the willingness-to-accept of 2,844 Facebook users to deactivate their Facebook accounts for four weeks, then randomly assigned a subset to actually do so in a way that we verified. Using a suite of outcomes from both surveys and direct measurement, we show that Facebook deactivation (i) reduced online activity, including other social media, while increasing oine activities such as watching TV alone and socializing with family and friends; (ii) reduced both factual news knowledge and political polarization; (iii) increased subjective well-being; and (iv) caused a large persistent reduction in Facebook use after the experiment. We use participants' pre-experiment and post-experiment Facebook valuations to quantify the extent to which factors such as projection bias might cause people to overvalue Facebook, finding that the magnitude of any such biases is likely minor relative to the large consumer surplus that Facebook generates.