Facebook Inc said on Monday it was raising wages for its US contract workers, such as cafeteria staff and janitors, to a minimum of $20 per hour in San Francisco Bay Area, New York and Washington D.C. and to $18 per hour in Seattle.

The company also said it will pay at least $22 per hour for content reviewers in the Bay Area, New York City and Washington, D.C., $20 per hour to those living in Seattle and $18 per hour in all other metro areas in the United States.

Facebook last raised minimum wages for contract workers in 2015 to $15 per hour in its bid to narrow the widening gap between the technology sector's elite and the lower-paid workers.

"In the years since, it's become clear that $15 per hour doesn't meet the cost of living in some of the places where we operate. After reviewing a number of factors including third-party guidelines, we're committing to a higher standard that better reflects local costs of living," the social media giant said in a blog post.

Last year, Amazon.com Inc raised its minimum wage to $15 an hour after facing harsh criticism over poor pay and working conditions. The online retailer said at the time that it would lobby Washington for the federal minimum wage to be raised. US corporations have been finding it tougher to attract workers, with US unemployment at its lowest level in nearly 50 years, while there has been growing political pressure on companies to pay workers a fair living wage.

Walmart, the world's largest retailer and the largest US private sector employer, pays workers $11 an hour at entry-level, while Target Corp said in April it would raise US minimum wage to $13 an hour.