Via WaPo (emphasis mine):
Former NSA and CIA director Michael Hayden stood on the pulpit of a church across from the White House on Sunday and declared Gmail the preferred online service of terrorists. As part of an adult education forum at St. John’s Episcopal Church, Hayden gave a wide ranging speech on "the tension between security and liberty."
During the speech, he specifically defended Section 702 of the Foreign Surveillance Intelligence Act (FISA), which provides the legal basis for the PRISM program. In doing so, Hayden claimed "Gmail is the preferred Internet service provider of terrorists worldwide," presumably meaning online service rather than the actual provider of Internet service. He added: "I don't think you're going to see that in a Google commercial, but it's free, it's ubiquitous, so of course it is."
The marketing slogans practically write themselves.