Key Points Question Are people willing to participate in research advertised on the internet, and is willingness to participate associated with type of study sponsor? Findings This mixed-methods survey and qualitative study of 914 respondents indicated that they were more likely to participate and share their social media data with researchers in university-led research studies than in studies conducted by the US federal government or pharmaceutical companies. However, only 49.3% indicated they would share their social media data at all. Meaning These findings indicate that researchers may face challenges in recruiting representative samples when recruiting from internet platforms., This mixed-methods survey and qualitative study investigates the association of willingness to participate in biomedical research and share social media data with source of study funding and online recruitment platform., Importance Using social media to recruit participants is a common and cost-effective practice. Willingness to participate (WTP) in biomedical research is a function of trust in the scientific team, which is closely tied to the source of funding and institutional connections. Objective To determine whether WTP and willingness to share social media data are associated with the type of research team and online recruitment platform. Design, Setting, and Participants This mixed-methods longitudinal survey and qualitative study was conducted over 2 points (T1 and T2) using Amazon’s Mechanical Turk (MTurk) platform. Participants were US adults aged 18 years or older who use at least 1 social media platform. Recruitment was stratified to match race/ethnicity proportions of the 2010 US Census. The volunteer sample consisted of 914 participants at T1, and 655 participants completed the follow-up survey 5 months later (T2). Main Outcomes and Measures Outcomes were (1) past experience with online research and sharing social media data for research; (2) WTP in research advertised online; (3) WTP in a study sponsored by a pharmaceutical company, a university, or a federal agency; and (4) willingness to share social media data. Opinions were solicited regarding the European Union’s General Data Protection Regulation statute, which came into effect between T1 and T2. Results Of 914 participants completing the first survey (T1), 604 (66.1%) were aged 18 to 39 years and 494 (54.0%) were female. Of these, 655 participants (71.7%) responded at T2. While 680 participants (74.4%) indicated WTP in biomedical research, only 454 (49.3%) were willing to share their social media data. Participants were significantly less likely to participate in federally sponsored (odds ratio [OR], 0.58; 95% CI, 0.51-0.64; P