Background Facebook is the leading social media site in the world with over 30 million active users in the UK, 50% of who log on every day. It enables promotion to be targeted to specified demographic groups and is cost-effective because advertisers only pay for users who click through to their website. Using Facebook we led an advertising campaign to men having sex with men (MSM).

Method The advert featured from 9th October to 31st December 2009 and offered a free rapid HIV testing and sexual health screening service to users actively logged on to their Facebook account. We targeted men interested in men, aged between 18 and 50 yrs, that were single or in a relationship and living within 50 miles of our city. Users clicking onto the advert were automatically diverted to our website's appointments section. Each click-through incurred a small fee (“Pay-per-click”).

Results The campaign targeted 30 580 registered Facebook MSM users. Over 3 months the advert appeared 2.86 million times. Up to 9750 individuals were exposed to the advert on any given day and it frequently featured several times a day to each user. In total, 872 (2.9%) registered MSM users clicked through to our website at an average cost of £0.55 per click. These were aged 18–24 (55%), 25–34 (27%), 35–44 (14%) and 45–54 (4%) years. The total cost of the campaign was £483. Facebook provided regular statistical reports which enabled us to monitor its effect, control our budget and prospectively tailor the advert accordingly.

Discussion This simple Pay-per-click Facebook campaign advertised to 8.5% of the UK MSM population (estimated UK MSM population is 3.6 million). Using Payment by Results income, only 0.3% (3–4) of clickers would need to attend our service to make the campaign cost-neutral. Whilst the functionality, diversity and social reach of Facebook is growing year on year, this presents a highly cost-effective opportunity for service/health promotion to hard to reach groups.