What would prevent Facebook from taking Google's path in a few years (assuming a similar scenario continues)?
Both Facebook and Google are gearing up to aggresively go after the mobile market. Apple is stagnating after Steve Jobs' passing and both companies taste blood inn the water.
The company has all the growth advantages while the M. Zuckerberg maintains the voting majority.
A lot of his decisions have pissed off the board but they've generally kept the company moving in a positive/profitable direction.
Google has a ridiculous amount of resources at its disposal and 2 very ambitious founder CEO's to drive the Alphabet parent company but, in reality, only have one groundbreakingly profitable product; advertising.
Mobile is taking over the web and Google's advertising capabilities on mobile aren't great due to technological limitations and the app store walled gardens.
AMP has the potential to put them in monopoly position in terms of web browser market share. They can leverage search engine ranking metrics to drive adoption.
Meanwhile ReactJS and Angular2 will are bringing 'universal' client-side webapps to the mainstream. Basically, the browser SPA frameworks can be used to create native mobile apps. I wouldn't be surprised if native mobile app development as a market collapses in the near future.
As for the rest, they seem to be aggressively pursuing infrastructure development. Customers hate the current broadband/wireless providers and anything that decreases internet usage is a threat to Google's bottom line.
Facebook and Google are driving in a smiliar direction as far as the web goes. Alphabet/Google just has much more resources and a much broader scope of where they plan to go.
In fact, it's to be expected. Microsoft -> Google -> Facebook -> Uber (ugh).
Each was very successful, and success breeds growth, which is not bad. However, as you keep growing, there's never any pressure to remove the lower-performers or focus. This leads to too many people for the (perceived) good projects, which leads to internal politics carried out in a destructive fashion. At that point, you're pretty much like every other company hated by their employees.