Uber is a startup that doesn't make money (actually loses quite a bit per ride), has just been rocked by a major scandal and doesn't posses any sort of tech or business process that is really unique. There is nothing that Uber does right now that another startup could just copy.
They lose money on each ride, and it's incredibly unclear how they will make money outside of trying to run taxis companies out of business by subsidizing fares with VC money and then jacking up fares when there is no more competition.
She also has lots of experience dealing / managing a strong-willed founder with all the equity control.
I really don't know anything about her personality and style, but if she's the super ambitious Mayer-type, I think she'll go for it (assuming she sees a path forward when she gets to see the behind-the-scenes numbers).
If you look at the numbers, all they need to do to reach profitability is increase fares by 7% without increasing the driver's cut.
I'm not suggesting they can just do that overnight without impacting their competitive position, but it's not "incredibly unclear" to me.
Raising fares 7% may get them to profitability, but does it get them to the point where a $70 billion valuation is justified?
Not saying Uber will go down the same path as FB but I am pointing out the flaw in your logic.