And in most of Facebook's offices, engineers and salespeople rarely interact. At the headquarters in Menlo Park, California, for example, the two work in completely separate buildings.

Most engineers can go their entire Facebook careers without ever meeting a salesperson, while only the best sales employees take the time to build relationships with engineers (who can come in handy whenever a client runs into a technical problem).

Engineering is the domain of CEO Mark Zuckerberg. Sales is COO Sheryl Sandberg's. Engineers like to work comfortably and quietly. Sales is loud. Engineering is young and very male. Sales skews older and more diverse.

There are two wings to the Facebook empire: sales and engineering.

But there are times when the two cultures clash. These clashes are rare, but highlight the tension Facebook faces every day: It must build products that users love and want to use, while simultaneously providing enough value to advertisers to keep the lights on.

One such clash came in 2015 at Facebook's New York offices, one of the most densely populated in the company. Although sales and engineering were kept on separate floors in New York, the two departments shared a common cafeteria.

Facebook's New York sales employees often brought big clients into the office for visits, taking them to lunch while on their visits. In that same space, engineers flexed their predilection for comfort.

"Engineers were going to cafeteria without their shoes on, and a sales person wrote, 'Hey, we have clients. Please keep in mind this is a professional environment,'" a former Facebook sales employee told CNBC.

"Everyone just treated it like a living room," said one former contractor. "That's pretty much what they encouraged … you wouldn't bat an eye if someone was walking around in slippers or socks."

"It was bizarre," said another former contractor.

The former sales employee recalled seeing engineers walk barefoot into the bathroom and then barefoot into the cafeteria.

The salesperson's post caused "a flare up" on the company's internal social network and in the New York office, according to five former employees and contractors who remember the incident, now known as "shoegate."

Sales and engineering employees left hundreds of comments defending both sides of the shoeless debate, and debating which side of the company was more important. The crux of the debate came down to: engineers build the product, but the salespeople keep the lights on.

Despite the battle of words, none of the former employees recall the company implementing any changes to the policy regarding shoes.

In 2016, Facebook leased space at another location in New York, giving the two divisions more breathing room to coexist, but at that moment in time, it was "a clash of cultures," a former employee said.

Facebook declined to comment.

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