Diversity reports issued by big Silicon Valley employers such as Apple, eBay, Facebook, Google and Twitter paint a portrait of a technology workforce that is overwhelmingly white and male. Where are the African-American and Hispanic workers, then? Filling low-wage jobs in security, janitorial and gardening services, according to a Working Partnerships USA report released Monday (PDF).

The report by the San Jose, California-based group said black and Latino workers make up 76 percent of landscape workers, 72 percent of janitors and 41 percent of private security guards throughout Santa Clara County, where Apple Inc. (NASDAQ:AAPL), Google Inc. (NASDAQ:GOOGL) and dozens of other tech companies are located.

It also noted the median hourly wage in the U.S. for janitors is about $11 per hour -- and $14 per hour for landscapers and security guards. Compare that to a median hourly wage of $63 per hour for software developers, who are also able to enjoy the free perks (such as exercise amenities and free food) that members of Silicon Valley’s underclass are not, as many of the low-wage workers are employed on a contractor basis.

“These contracted service workers -- not counted on tech companies’ official employment rolls and rarely mentioned in the public discourse -- constitute the Silicon Valley tech industry’s ‘invisible workforce,’” the report said.

While secretaries and artists who get rich from initial public offerings are part of Silicon Valley lore, the reality is its unskilled workers are almost entirely in a contract workforce that doesn't benefit directly from the outsize wealth generated there.

“These twin dynamics -- lack of high-end tech jobs, and lack of adequate wages in contracted service jobs -- have a profound impact on the communities that are left behind by Silicon Valley’s flagship industry,” the report said. “Once a region with a range of workforce opportunities, a strong middle class, and the prospect for working families to get ahead, the Valley has emerged from the recent ups and downs as a segmented economy with an increasing number of working families struggling to make ends meet.”

The entire workforce of Santa Clara County -- historically home to operations of Apple, Facebook Inc. (NASDAQ:FB) and Yahoo Inc. (NASDAQ:YHOO) -- is 28 percent black or Hispanic, a figure that indicates the disparity in employment among major technology players can’t be blamed on the demographics of the surrounding area.

Google’s own numbers show the company is 59 percent white, 34 percent Asian, 2 percent Hispanic and 1 percent black. Facebook is 3 percent Hispanic and 1 percent black; Apple is 7 percent Hispanic and 6 percent black; and Twitter Inc. (NYSE:TWTR) is 3 percent Hispanic and 1 percent black.

Online auction company eBay Inc. (NASDAQ:EBAY), with a mostly white workforce that is 42 percent female, as well as 7 percent black and 5 percent Hispanic, has one of the most diverse workforces in the tech industry.

Tech-company executives have repeatedly promised to increase diversity among their workforces, with Apple CEO Tim Cook saying he was “not satisfied with the numbers” in his company’s diversity report.

Before he was laid off, Michael Johnson, a 52-year-old black man, said he worked 15 years in tech, primarily as a systems engineer in telecommunications. Since then, Johnson said that he had to find work as a security guard. In a conversation with the Wall Street Journal, he was less circumspect about the problem in Silicon Valley.  “Talk is cheap,” he said. “They are two different worlds.”