BEVERLY HILLS, Calif. -- Like her counterparts who have arrived at other older companies, Yahoo CEO Marissa Mayer prides herself on having pushed her team and the products they build into the world of mobile. For Yahoo the march to mobile always appeared to come begrudgingly over years of fits and starts."When I came to Yahoo one of the weaknesses was that mobile was everyone's hobby and nobody's responsibility," she said earlier this week at the annual 4As conference. The company now has close to 500 product engineers and managers working on mobile and "you can feel that in terms of what we're developing," she says.Mobile Half of All Monthly Active UsageYahoo is tracking at least 400 million monthly active visitors on mobile today, up from 160 million a year ago, Mayer says. That's out of 800 million monthly active users on Yahoo's core network, an audience that's also up 20 percent year-over-year.While the company still declines to publicize its daily active user numbers, a metric widely adopted by its competitors, Mayer says Yahoo maintains one of the largest audiences in the U.S. with more than 200 million users visiting the site at least once a month."We're also really focused on time spent. We've seen time spent growing on the core Yahoo network as well as Tumblr," says Mayer, adding that Tumblr is approaching a 300-percent year-over-year gain in time spent on mobile alone.Outside of mobile, Mayer is also directing the company's attention and making large investments in search, mail, social media, native advertising, video and digital magazines.Yahoo as a Digital Daily HabitIndeed, Mayer has talked about making Yahoo a key digital daily habit for a growing user base ever since she joined the company in the summer of 2012. Even before she arrived, Yahoo already encompassed finance, news, sports, entertainment, email and search, among others. "All things that people have done every day for the last decade or two," she says.While the leading brands and advertising agencies in attendance certainly care about how many users their ads might reach on Yahoo, Mayer says she is acutely aware that "what you want to understand from an advertising perspective is who you're reaching."One way to gain those insights for advertisers is the persistent identity. Mayer has decided to remove alternative log-ins for the site and only support Yahoo accounts going forward, a move that she says gives Yahoo the best chance to do right by its users and advertisers. Persistent IDs also enable content customization and resolve many potential privacy and security concerns, she says."Streams are a modern paradigm. People are becoming more and more accustomed to this never-ending stream of information," says Mayer. "What a stream does is allows us to present as much information as the user asks for and present it in a personalized way."Those streams also play a big part in Yahoo's burgeoning suite of programmatic advertising options, including an ad exchange and ad manager that will be launching in the coming months, she says calling the offering "the first of its kind in terms of a unified marketplace."Tumblr Ads Deliver Promising NumbersOf course no interview with Mayer is complete without getting her latest thoughts on Tumblr, a company Yahoo acquired for $1.1 billion last May, and moderator GroupM's Chief Digital Officer Rob Norman did not disappoint."One of the most exciting pieces about Tumblr is the fact that it's such an amazing canvas for storytelling," she says. "Tumblr is the home of the world's creators, but there's nothing as creative as advertising."So in that forum of creativity with more than 150 million blogs today, how are ads performing? "The average post on Tumblr gets re-blogged 14 times," she says. "The average sponsored post on Tumblr gets re-blogged 10,000 times."Matt Kapko covers social media for CIO.com. Follow Matt on Twitter @mattkapko. Email him at email@example.com Follow everything from CIO.com on Twitter @CIOonline and on Facebook.