Security is so hard that you can't find everything. One of the dirty secrets of the security industry that nobody likes to talk about is that when you do a pentest, you're deemed secure, but it's overwhelmingly likely that the pentesters didn't find all the vulnerabilities.
Security breaches are rare, which is why this system works. But it's not malicious -- it's just very, very hard to be secure all the time. Security isn't even a priority in most cases.
I get that you're saying Facebook has resources. But with resources comes scale. How many programmers do you suppose worked on that feature? It could be one or two out of a dozen assigned to that arm of the project. Or it could be a dozen involved in a multi-month refactoring of spaghetti code. Or it could be one overworked person who threw in a "fix" before crawling home to bed. FB hires exceptional people, and even exceptional people ship bugs.
By the way, two weeks is a tiny amount of time. A real-world pentest takes about that long to book, unless you already have pentesters on retainer. I'm sure Facebook does, but as I said, pentesters can't catch everything.
Good security folks are valuable for a reason. Good luck to your efforts.
A bug in the software, discovered late last year, resulted in the personal profiles of content moderators automatically appearing as notifications in the activity log of the Facebook groups, whose administrators were removed from the platform for breaching the terms of service. The personal details of Facebook moderators were then viewable to the remaining admins of the group.
So nobody noticed that or fixed it for 2 weeks (edit: the article actually says that the bug was active for 2 weeks before being discovered and not fixed for another 2 weeks, and was retroactively exposing employees for more than a year - is that security)? Thousands of employees and nobody is checking for that?
Come on. We praise Elon sending things into space but FB can't do some basic error checking? Something doesn't add up.