If you are an 18- to 40-year-old living in metropolitan Sydney who has "liked" music festivals and clubbing on Facebook, you may have been identified by a Sydney law firm as a potential drug user.

Using Facebook's targeted advertising, Ly Lawyers promotes criminal law services to a potential pool of young drug users who share their interest in music and clubbing on Facebook.

Revellers at the Field Day music festival held at The Domain. Credit: Sahlan Hayes

"Busted with possession or supply of drugs at a festival? Call Sydney's best drug lawyers!", one advertisement says, accompanied by a picture of a hand chopping up a white powder.

The advertising ran for days after Sydney's music festival Field Day, where police made a record 214 arrests for possession of illegal drugs including ecstasy, cocaine, ice and cannabis.