Mark Zuckerberg, chief executive officer of Facebook Inc. Tony Avelar | Bloomberg | Getty Images

Some former Facebook employees say their phone is ringing a lot more in the last two months. On the other line: former Facebook colleagues asking about job openings or looking for a reference. This type of behavior is normal at most companies. But according to a half dozen former employees, all of whom left in the last year or two, it's a major change in behavior at Facebook, which had long been known around Silicon Valley as the company that no one leaves. These people requested anonymity as none is authorized by Facebook to talk about their time there or interactions with employees.

The shift could be an early warning of recruiting and retention challenges for Facebook after a turbulent year. In 2018, the company has faced public questioning at multiple congressional hearings, scandals around third-party abuse of user data and public relations practices and flat or declining user growth in key markets. It's also seen its stock drop nearly 40 percent from July. The stories from former employees are only anecdotal at this point, and there's no firm data showing a significant uptick in departures or employee dissatisfaction. On Glassdoor, a site where workers anonymously review their employers, Facebook is among the best-rated tech companies, with a satisfaction rating of 4.3 out of 5. However, that rating has fallen noticeably during the last year, with a particularly sharp drop in the last few months.

Even if Facebook employees are starting to consider other options, that's not uncommon as hot tech companies mature. Around 2010, Google saw a wave of engineers and executives leave for greener pastures, including Facebook. Microsoft faced a similar exodus in the early 2000s, captured most saliently by the anonymous employee blog Mini-Microsoft. "Our retention rate continues to be very strong," said Anthony Harrison, a Facebook spokesman. "Everyone at Facebook is focused on making a positive impact in the world, and on working on hard challenges that matter." But several former employees who spoke to CNBC believe that the wave of scandals and falling stock price are spurring more people to consider leaving for the first time. "There's new things coming out every day," said one former company executive. "It's a quite somber atmosphere right now at the company."

'They're just burnt'

Employees have a tradition of posting photos of their badges on secret Facebook groups for alumni of the company when they quit, another former Facebook manager said. "I've been seeing a lot of badges lately," he said. This manager also said he'd heard recently from a couple of current Facebook employees, including one individual who has been with the company for more than seven years but expressed that he has finally burned out. "A lot of people want to do something different," he said. "They're just burnt." A former Facebook recruiter said he has heard from more than 30 current employees in the past year, including approximately 15 in the past two months. Most of these employees say "'My manager sucks, and I need to look for something new. Do you know of any new opportunities?'" the recruiter said.

"They're coming to ask 'What are you seeing in the market across Silicon Valley?'" the recruiter said. Some former employees cited a broader change in the culture as well. In the past year, the company has grown from nearly 23,200 employees in September 2017 to more than 33,600 employees a year later, according to the company's latest financial filings. With that growth has come increased bureaucracy and an increase in a top-down management style, one former Facebook manager said. Whereas previously Facebook had a start-up environment where colleagues felt everyone had each other's backs, there is now more politics and more grandstanding, another former manager said. "There's a lot of people who succeed more by how things looked than by the work they were doing, and there were people who were let go that were incredibly well-respected and it was because they weren't playing the politics game," one of the former managers said. One former Facebook engineer said he has been contacted by about a dozen Facebook employees since leaving the company this summer, saying they were thinking about leaving the company or inquiring what his experience has been like since his own departure. Just before speaking with CNBC, another Facebook employee called him to ask for advice on clearance for setting up a start-up while remaining at Facebook, this person said. "Overall, I've seen an uptick in people either looking for other activities or dipping their toes outside the Facebook pool," he said. In the past, Facebook employees who had been with the company for only a year or two but were unhappy with their roles were likely to request a team switch. Now, employees in that situation are simply looking to leave the company, the former engineer said. "This time around far more people are immediately jumping instead of switching teams," the former Facebook engineer said.

Leaving the right way is important