Objectives: Recruitment of sufficient sample size into clinical trials is challenging. Conventional advertising methods are expensive and are often ineffective. The effectiveness of Facebook for recruitment into blood pressure clinical trials of middle-to-older-aged people is unknown. This study aimed to assess this by comparing Facebook advertising with conventional recruitment methods from a retrospective analysis within a clinical trial. Methods: Conventional advertisements (newspaper, radio and posters) were employed for the first 20 months of a randomized controlled clinical trial conducted in three Australian capital cities from Tasmania, Queensland and the Australian Capital Territory. With dwindling participant recruitment, at 20 months a Facebook advertising campaign was employed intermittently over a 4-month period. Recruitment results were retrospectively compared with those using conventional methods in the previous 4 months. Results: Compared with conventional recruitment methods, Facebook advertisement was associated with a significant increase in the number of participants recruited in the Australian Capital Territory (from an average 1.8-7.3/month; P<0.05). There was also an increase in Tasmania that was of borderline significance (from 4.0 participants recruited/month to 9.3/month; P=0.052). However, there was no effect in Queensland (from 6.0 participants recruited/month to 3.0/month; P=0.15). Facebook advertisement was associated with a significant decrease in the age of participants enquiring into the study (from 60.9 to 58.7 years; P<0.001). Conclusion: Facebook advertising was successful in helping to increase recruitment of middle-to-older aged participants into a blood pressure clinical trial, although there may be some variability in effect that is dependent on location.