What is the relationship between Dick Costolo and Twitter in the context of the input sentence. Choose an answer from: product/material produced; manufacturer; distributed by; industry; position held; original broadcaster; owned by; founded by; distribution format; headquarters location; stock exchange; currency; parent organization; chief executive officer; director/manager; owner of; operator; member of; employer; chairperson; platform; subsidiary; legal form; publisher; developer; brand; business division; location of formation; creator.

Even as the Street was bemoaning its MAU problems and then-CEO Dick Costolo was trying anything and everything to turn its 500 million monthly visitors into actual users, Twitter bulls could always point to sales growth.

Answer: employer