Before Marissa Mayer was made CEO, the name Yahoo had mostly faded from the mainstream. This did not mean that people had stopped using the service; in fact, usage was not down, just not something people talked about. Of course, anywhere people are using a data collecting service, the government takes notice, even when the press does not.
One way the US government took notice of Yahoo in the years before Mayer remade the company's image in the public was in secret, sealed data requests. These requests, and the ones issued to other tech companies, became the basis for a little program named PRISM. PRISM was the program that Edward Snowden, an NSA contractor, sacrificed everything to make public.
What is different about the Yahoo requests is the company's response. When the requests came in, Yahoo declined to turn over the data without an issued warrant; they claimed the warrantless data collection to be "unconstitutional and overbroad," bringing the matter to court. Unfortunately for Yahoo, and the American public, the case did not go well. Yahoo General Counsel said,
Our challenge, and a later appeal in the case, did not succeed. The Foreign Intelligence Surveillance Court (FISC)... ordered us to give the U.S. Government the user data it sought in the matter.
Despite the court loss, Yahoo still refused to turn over the data, under the same claims of unconstitutional behavior. That's when the government threatened fines: $250,000 per day for non-compliance. They also used the sealed results of the private court case and fines to threaten other companies. When complete, the PRISM program was collecting information constantly from the likes of AOL, Apple, Facebook, Google, Microsoft, Skype (before and after Microsoft purchase) and YouTube.
All because of Yahoo's loss in court, the US government was able to create the PRISM program, and stock it with data about people all over the world: US citizens and not. It is an interesting notion that Yahoo could have had so much of a secret effect on the globe while having seemingly no public effect.
Marissa would see to it that Yahoo would continue to make a mark in the PRISM case after the Snowden leaks came out. She released a transparency report showing as much information as she was legally permitted to about the nature and targets of the data requests, dating back to 2007. Because of this, other companies on the list also released reports, including Microsoft/Skype.