At first, it promisees safe browsing (but why FB only? Google os no better). But than... It's a very frustrating thing to use. If you go to blocked site you can't go back - history of the tab is erased. The same if you go to unblocked side from restricted one. Why the f?!
The extension is highly powerhungry, it leads to some sites hanging. When you disable it all the restricted sites close WITH NO WAY TO RESTORE! And the same is when you enable the addon.
I could endure that, but it's too much for the thing protecting me from FB only. Google, Yahoo, and many more "Good companies" spy on us, what's the reason to defend us from only single one?