Utilize the input text as a context reference, choose the right relationship between Dick Costolo and Twitter from the options. Options: 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.

Twitter co-founder Dorsey, who took over last month from Dick Costolo in the midst of criticism over the company's strategy, warned on Tuesday that Twitter was unlikely to see sustained growth in monthly active users (MAU) until it can reach the mass market.

Answer: employer