"Go out on a limb" suggests that the author is taking a risk by doing or saying something. In this case, you can read it as "I am going to put myself out there (at the risk of getting unpleasant feedback) and declare Facebook is the most intense website in the world.". Kinda like... 冒天下之大不韪 but not as intense. I would even say this is just a mild exaggeration.

Go out on a limb is like "I'm going to take the risk of saying this but.." if that makes sense.