Apparently social media and many consumers have finally gotten wind of the fact that Costco (NASDAQ:COST) is actually a positive, socially responsible retailer. I've seen quite a few infographics floating through Facebook recently, touting Costco's good citizenship as compared to Wal-Mart (NYSE:WMT), the latter of which has been a lightning rod for recent criticism.

This is why I bought shares of Costco for my Prosocial Portfolio two years ago, even though many were probably surprised I'd call it a socially responsible company at the time. Now that more people get it, I rest my case -- and remain bullish for the future.

Former CEO Jim Sinegal -- who retired from the CEO role nearly a year ago -- has been one of the few American chief executives to provide an example of how to build a company that makes employees, customers, and true long-term shareholders proud even during the most turbulent times. Costco does indeed pay its employees more than industry rivals like Wal-Mart and Target (NYSE:TGT). (Believe it or not, Target's pay is actually a bit worse than Wal-Mart's, so there's yet another surprising factoid for the day.)

Sinegal, who was modestly paid for a CEO of a well-known company, has long provided workers with many benefits including health care, and resisted Wall Street's pressure to cut those benefits even during the recession.

Here's Sinegal's response to Wall Street's short-term profit pressure in a Fast Company interview from 2008: "You have to recognize -- and I don't mean this in an acrimonious sense -- that the people in that business are trying to make money between now and next Thursday. We're trying to build a company that's going to be here 50 and 60 years from now."

Rethinking the strongest form of capitalism
Many investors find the idea of companies treating certain employees well the antithesis of "good" form in capitalism (however, many of these same investors seem to have no problem with overpaid, non-performing CEOs, interestingly enough). In economic reality, this attitude is detrimental to long-term business health, for even more reasons than that simple mean-spiritedness results in low worker morale.

According to a 2012 paper by MIT's Sloan School of Management professor Zeynep Ton, published in the Harvard Business Review, companies that treat employees well may have all kinds of competitive advantages over their more stingy counterparts.

Ton observed: "Highly successful retail chains -- such as QuikTrip convenience stores, Mercadona and Trader Joe's supermarkets, and Costco wholesale clubs -- not only invest heavily in store employees, but also have the lowest prices in their industries, solid financial performance, and better customer service than their competitors."

She also noted that companies with well-trained, extremely motivated employees tended to unlock efficiencies, which of course is one of the most essential things capitalism is supposed to do.

Ton's study is no knee-jerk takeaway, either; she's been studying the area for 10 years.

Hey, shareholders: Think stakeholder value
My recent argument in favor of the stakeholder value model -- which Costco exemplifies -- is the theory that companies that treat their customers, workers, and suppliers well are the best companies to invest in and hold for the long term.

Along these lines, companies that have bad reputations are good to avoid, in my opinion. Last year, a study by 24/7 Wall Street using reviews pointed to "Worst Companies to Work For," with retailers like RadioShack (NYSE: RSH) (major problems reported with its commission/compensation structure, described by one reviewer as "a big joke," and long hours) and Dillard's (NYSE:DDS) (complaints included poor sales incentives -- with employees judged on hourly sales -- and an unpopular founding-family CEO) landing high on the list. No thanks.

I'm still looking for more socially aware picks for my real-money Prosocial Portfolio, but so far, my Costco purchase has been a long-term winner, with a 46% gain; it's a company whose shares I continue to feel comfortable holding for the long term. Shareholders (some of whom may be employees, since Costco also has an easy stock purchase plan for its workers) can also appreciate today's announcement of its one-time $7 special dividend.

I'd like to see a temporarily lower price to buy more Costco shares (and I'd advise potential investors to wait for weakness before buying in, too). Regardless, the long-term bottom line is that Costco's a gold-standard company to hold through thick and thin. It deserves a place in positive, responsible portfolios.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.