free rider vs sponger

free rider

noun
  • A worker at an open shop who is able to enjoy the benefits negotiated by a union without being a member of the union. 

  • Someone who obtains goods or services legally without paying. 

  • One who obtains benefit from a public good without paying for it directly. 

sponger

noun
  • A device for sponging cloth by means of a perforated adjustable cylinder. 

  • One who uses a sponge. 

  • A parasitic hanger-on. 

  • A person or vessel employed in gathering sponges from the sea. 

How often have the words free rider and sponger occurred in a corpus of books? (source: Google Ngram Viewer )