John Herrman at BuzzFeed explains that the Facebook/Google/Microsoft/etc. denials of working with PRISM have two things in common. One is the suggestion that the government was not given "direct access" to their servers, which could basically be those companies making use of "a technicality." The other assertion that nearly all the companies made was that they had never heard of PRISM. Herrman explains that there's "no reason the companies involved would have necessarily heard about it under that designation." You should go read the full story. There's a lot of supposition in there, but it's at least a recognition of a pattern that may eventually provide some concrete answers.