A true statement is that Facebook's security teams have been shifted around in several reorgs. A false statement is that Facebook has dissolved its security teams. The latter is a mischaracterization of the former, because while some security staff have left Facebook for a variety of reasons, the company is not deliberately reducing its security staff nor encouraging their departure. It still employs a huge number of engineers specializing in every major domain of information security.
If you'd like evidence that Facebook is expanding its security presence, you can take a look at its careers portal. It's aggressively hiring security staff in satellite offices that previously weren't focus areas for security engineering.
In my opinion, Alex Stamos' company memo gives a clearer picture of what's happened in Facebook's security org recently.[1] You should read that in addition to media reports.
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1. https://www.buzzfeednews.com/article/ryanmac/facebook-alex-s...