The better question is, why are people wasting their time trying to invent a federated Facebook instead of a federated Google? People already have lots of ways to share pictures of their cats. But they only have one way to find information on the internet, and if the people running that monopoly did something like compromise the quality of that service to promote their sharing-cat-pictures product, we'd be screwed.
>But they only have one way to find information on the internet
That isn't remotely true. There are dozens of alternatives; Google's simply better. That said, the difference in quality is not so pronounced as it used to be.
And yet better still, why implement a federated anything when you can hypothetically have a framework off of which you can spawn a whole host of distributed integrated web services.
Or maybe that's just impossible, haven't really done the research.
