Remember all the "act-surprised-and-disgusted" posts of these involved companies, right after Snowden had started his revelations? Personally, I remember Zuckerberg's "What the F#ck" post very well.
They were right to be surprised and deny it. The initial reporting in the Guardian and Washington Post all said that NSA had "direct access" to the servers of all of these companies. After a few days, it was revealed that the NSA did not, in fact, have direct access to their networks, but instead had a system in place to retrieve data provided by these companies under court order.[1]
The Washington Post quietly revised its article without issuing a formal correction[2], but to date Glenn Greenwald has yet to retract his statement the NSA has direct access to their networks.
None of these individual companies would have known about the NSA end of the webservice NSA calls PRISM.
Does the Zuck have a TOP SECRET clearance with read-ins to all the special access programs that would be needed to know about PRISM? Somehow I doubt it.
There was acting going on with those responses alright, but the acting more about pretending they didn't understand how powerful automating warrant compliance was, instead simply playing along to the crowd of hacktivists.
But none of these companies would have known about the NSA side, all they'd have seen would be the company's end of the operation, which would be nothing more than an archival tool of a user's data within a certain filter set followed by an upload tool to some NSA-controlled server. They wouldn't have even known what NSA calls the whole operation.
Weren't they justified in that reaction? As best I have been told and can recall, the CEOs recognised neither the "PRISM" name (presumably because it was an internal label which the government hadn't shared with the webcos?) nor the initial description in the press of the PRISM program (apparently because it was highly inaccurate). In that case it's easy to see how their reaction could be genuine.