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.

But Dorsey, who also founded Twitter in 2006, returned to the role of Twitter CEO in June this year after Dick Costolo said he planned to step down.
employer