Well they did say that Cassandra is better than all the others - especially when compared to HBase - on nearly all measurements (except for high write scenario latencies.)
This makes the decision of Facebook to go with HBase for their new messaging platform back in 2010 all the more strange. Though that was two years ago so things might have changed in Cassandra's favor since then.
This was last year, and the recruiter was an old timer, so to speak; he wasn't new to FB. He was recruiting for the software engineering positions.
Really, are you surprised that there are federal agents on the premises, or just upset at the thought that FB users' privacy could be compromised in such a situation? To the degree that FB forum and groups ToS conforms with federal law and moreover seeks to go above and beyond the letter of the law then FB in this regard could be thought of as an extension of the federal government in such matters with regards to its own users. In this light it is no more relevant whether federal agents look at private or group communications, as opposed to bona fide FB employees. It doesn't matter. This is a private company; the first amendment doesn't apply. In any case, FB as a company isn't exactly widely respected for its position on its users' data, nor is it known to play nice with its users' privacy.
Let's not forget that of the companies whom the federal government sought to extract users' accounts' data from with regards to the Wikileaks issue 18 months ago, only Twitter publicized the compelled exposure of its users' data. Facebook happily gave the feds all the users' info they sought without telling us about it.
Although IIRC you have said that you've worked with the FB spam and fake account / phishing detection team, FB is a large enough organization at this point that the team responsible for complying with federal mandates and managing FB's relation with federal law enforcement is separate. Tao Stein and team have no bearing on this particular matter, nor should they care that real users happen to talk about and discuss real issues that federal agents think should be taken down. This is a separate matter entirely from what you've worked on.
I was once informed by an FB employee that federal agents are ensconced at the FB premises to monitor users' communications and shut down / censor FB groups and venues for "hate" speech and terroristic threats.
This makes the decision of Facebook to go with HBase for their new messaging platform back in 2010 all the more strange. Though that was two years ago so things might have changed in Cassandra's favor since then.