Read an interesting article about this subject. Interesting not in the sense of costs or efficacy but mostly on how they did it. It’s done for an epidemiological study on a mother child cohort. They wanted to include pregnant women for their study with facebook beside other forms of recruitment such as: active collaboration with health personnel involved in pregnancy and childbirth who distribute leaflets or introduce the study to pregnant women, a website, links to the website on other websites dedicated to pregnant women and participation in online pregnancy related forums.
What they did with facebook was creating a weekly updated facebook page and an advertising campaign on facebook. The advertisement was displayed on personal facebook pages of women aged 18 to 45 with an IP address in Italy whom were selected by 2 keywords pregnancy and delivery )in Italian). The page got a lot of fans (167-450) and during the campaign of 5 weeks 8 participants came to the study through facebook.
Compared to their other forms of recruitment it was not cheap but than again more spin off from the facebook page could not be measured. Talking money, the advertisement costs were 200 euros.
Richiardi, L., Pivetta, E., & Merletti, F. (2012). Recruiting Study Participants Through Facebook Epidemiology, 23 (1) DOI: 10.1097/EDE.0b013e31823b5ee4