The rise of social media websites (e.g., Facebook) and online services such as Google AdWords and Amazon Mechanical Turk (MTurk) offers new opportunities for researchers to recruit study participants. Although researchers have started to use these emerging methods, little is known about how they perform in terms of cost efficiency and, more importantly, the types of people that they ultimately recruit. Here, we report findings about the performance of four online sources for recruiting iPhone users to participate in a web survey. The findings reveal very different performances between two types of strategies: those that "pull in" online users actively looking for paid work (MTurk workers and Craigslist users) and those that "push out" a recruiting ad to online users engaged in other, unrelated online activities (Google AdWords and Facebook). The pull-method recruits were more cost efficient and committed to the survey task, while the push-method recruits were more demographically diverse.