Basically purchase aspiring start-ups that hit some troubled time scaling (Polyvore seemed to be running into some monetization issues). By putting the digital properties into Yahoo's network, the acquired company get lots of free traffic + better monetization (Yahoo already has a great brand sales department + the data Yahoo might have on people could make more engaging ad programmes).
My bet is Marissa is trying to build more driven, savvy, smarter leadership by buying it and promising influence.
They've finally launched a mobile app. Looks nice, if it gains traction, then the new Yahoo! has arrived.
Yahoo! is copying Facebook, not 100% but in the way of humility. Mayer's Yahoo is finally pivoting to mobile and to social because that's the 'new homepage'. Look at all the social media addicts, hoping for likes and comments. Yahoo! has seemed to have learned the error of pride, they are now trying the meritorious way. I think it will pay off.
(I view FB's strategy as Mobile first. ~70% of their revenue is now mobile. Which is 4x what Yahoo! currently makes.)