Americans are increasingly turning to social media for political information. However, given that the average social media user only clicks through on a small fraction of the political content available, the brief article previews that appear in the News Feed likely serve as shortcuts to political information. Yet, in addition to sharing political news, social media also allow users to make their own comments on news posts, comments which may challenge or distort the information contained in the articles. In this paper, we first analyze how social media posts on Twitter and Facebook differ from the actual content of their linked news articles, finding that social media comments regularly misrepresent the facts reported in the news. We then use a survey experiment to test the consequences of these information discrepancies. Specifically, we randomly assign individuals to read a full news article, a news article preview post (as seen on Facebook), or a news article preview with misinformative social commentary attached. We find that individuals in the social commentary conditions are more misinformed about the featured topic, tending to report the factually-incorrect information relayed in the comments rather than the factually-correct information embedded within the article preview.