There are over 900 million users on Facebook and even attracting business from 1% of them is quite significant. The advertisements are programmed to reach specific and targeted audience depending on location, age, personal interest, job profile, etc. Instead of making your ads appear for everyone, The ads are made for users who have expressed interest in dental services. They also target local customers who would prefer a dental service nearby.
For More Info : http://socialhi5.com/blog/benefits-of-facebook-advertising-for-dentist/