This is off course a legitimate question. I tried to really summarize it by saying: "loosing one level will cause the collapse of the others".

The browser is for the end user an important base because that is where they get directed to surf and target ad spending etc. By inserting functionality like a start page or search engine, the behavior is influenced a lot. You can also push own services and products directly.

The current status of the browser is also that it can replace the OS. (See Google Chrome OS)

Google made a browser so that Google products wouldn't be hindered by whatever the state of the browser market was. And Google was the Internet for lots of lay users.

Now since Facebook is the Internet for lots of lay users, I suppose they might use the same driving logic?