Cancer misinformation posts are shown with a prompt above the post caption that read, "This post has been flagged as false and potentially harmful. If you want to report, click the flag below."

Measured with 1 self-reported survey item: "How willing are you to dislike (thumbs down) this post?" Responses ranged from "not at all" (coded as 1) to "extremely" (5). Assessed 15 times. Assessed for 2 cancer misinformation posts and 1 cancer support post at each timepoint approximately 2 business days apart, starting on the day of enrollment.

Measured with 1 self-reported survey item: "How willing are you to like (heart) this post?" Responses ranged from "not at all" (coded as 1) to "extremely" (5). Assessed 15 times. Assessed for 2 cancer misinformation posts and 1 cancer support post at each timepoint approximately 2 business days apart, starting on the day of enrollment.

Measured with 1 self-reported survey item: "How willing are you to comment on this post?" Responses ranged from "not at all" (coded as 1) to "extremely" (5). Assessed 15 times. Assessed for 2 cancer misinformation posts and 1 cancer support post at each timepoint approximately 2 business days apart, starting on the day of enrollment.

Measured with 1 self-reported survey item: "How likely are you to share or tell someone about this post?" Responses ranged from "not at all" (coded as 1) to "extremely" (5). Assessed 15 times. Assessed for 2 cancer misinformation posts and 1 cancer support post at each timepoint approximately 2 business days apart, starting on the day of enrollment.

Measured with 1 self-reported survey item: "What type of comment would you write on this post?" Response options were "comment to agree" (share), "comment to disagree" (intervene), "other," or "I would not comment." Assessed 15 times. Assessed for 2 cancer misinformation posts and 1 cancer support post at each timepoint approximately 2 business days apart, starting on the day of enrollment.

Measured with 1 self-report survey item: "Who would you share about this post with?" Response options were "someone with cancer," "someone who is a cancer caregiver," "health-care provider," "other," or "no one." Assessed 6 times. Assessed for 2 cancer misinformation posts and 1 cancer support post at first timepoint and last timepoint approximately 2 weeks apart.

Measured with 1 self-report survey item: "How would you share about this post?" Response options were "private message on social media," "text message," "in person," "repost on your social media," "other," and "none of these ways." Assessed 6 times. Assessed for 2 cancer misinformation posts and 1 cancer support post at timepoint 1 and timepoint 5 approximately 2 weeks apart.

Measured with 1 self-reported survey item: "In the last 7 days, did you take action against false and potentially harmful cancer posts from social media? Select all that apply." Response options included muted accounts, flagged posts, disliked posts, commented to correct posts, chatted with others to disagree with posts information, encouraged someone to flag or report a posts, other or did not do anything. Assessed 1 time. Assessed at timepoint 4 approximately a week after the day of enrollment

Measured with 1 self-reported survey item per reported interpersonal intervening behavior: "Who did you [intervening behavior]? Select all that apply." Response options were "someone with cancer," "someone who is a cancer caregiver," "health-care provider," "other," or "no one." Assessed up to 2 times. Assessed at timepoint 4 approximately a week after the day of enrollment.

Measured with 1 self-reported survey item per reported intervening behavior: "What was the [source for intervening] about? Select all that apply." Response options were "to adopt a new mindset," "diet and nutrition advice," "physical activity advice," "advice to stop clinical care," "to seek treatment outside the US," or "other." Assessed up to 6 times. Assessed at timepoint 4 approximately a week after the day of enrollment.