Why does the idea of Facebook as keyserver worry you? How is it worse than using another keyserver?

I don't need a MIT account to use MIT's key server. Facebook has the tendency to make things available only behind their wall.

Why couldn't you use both?

There's perhaps the risk of it becoming the major anchor in the web of trust. That said, you're probably right with respect to the other keyservers being no better.

Precisely why they should be adopting Google's End-to-End system. Yahoo is already supporting it. Also it would be better if everyone went with the same (solid) standard than doing their own thing.

Can you tell me more about the state of End-to-End?

It works, and should still be used with caution.