Barnacles feed on small organic particles and plankton that they catch on their cirral nets.  In low rates of water flow the cirri may sweep-feed; in higher rates of flow the cirri are simply extended into the current. 

Analyses of stomach contents of several species of west-coast acorn barnacles reveals the presence of diatoms, seaweed bits, small gastropods, crustaceans, various larvae, sand grains, and detrital debris of less than 2um in size.  Since the cirral filtering basket cannot capture such small particles, the author of this review article suggests that they may be enmeshed in glandular secretions at the base of the cirri, and then carried to the mouth for acceptance or rejection.  Little species-specific dietary preferences are evident in the data.  Barnes 1959 Can J Zool 37: 231.

NOTE  these include Balanus glandula, B. nubilis, B. rostatus, Tetraclita squamosa (California only), and Semibalanus cariosus

Giant barnacle Balanus nubilis with
cirri extended for feeding 2X