Facebook Inc. has invested heavily the past few years in efforts to hire a more diverse workforce. But those efforts don't guarantee a socially accepting workplace.

Chief Executive Officer Mark Zuckerberg issued a companywide memo Thursday chastising employees who, despite his earlier warnings, have been crossing out the phrase "black lives matter" and writing "all lives matter" on Facebook's free expression walls at its Menlo Park, California, campus. In the memo, published earlier by Gizmodo, Zuckerberg notes that he has never had to make rules about what can be written on the chalkboards -- but he considered these acts "malicious."

"This has been a deeply hurtful and tiresome experience for the black community and really the entire Facebook community, and we are now investigating the current incidents," he told employees in an internal company posting. He explained that "black lives matter" doesn't mean other lives don't. "It's simply asking that the black community also achieves the justice they deserve."