Facebook claims that frictionless sharing makes sharing easier. They've improved the usability of sharing by taking away the friction.
So let's look at it from a usability perspective.
This is an oversimplification, but we can think of frictionless sharing as an attempt to replace something like this:
With something like this:
Instead of requiring the user to confirm every single article they choose to share, just give them a one-time dialog that enables them to share everything down the road.
That's a lot less work for the user, right?
Well, no, not really. Because in the past the user only had to decide whether to share something they just read, but now they have to think about every single article before they even read it. If I read this article, then everyone will know I read it, and do I really want people to know I read it?
That creates more friction, not less.
And let's not forget the friction the user experiences as they browse around the Web. Now they have to remember which sites are automatically sharing what they read. Did I allow a Facebook app to share what I read on this site? I don't remember, so I'd better not click that link.
So frictionless sharing isn't frictionless after all. All it does is trade the small friction of having to choose what to share with the large friction of having to think about whether what you're about to do will be shared.