…it depends on where in the world you are. Certainly in many parts of the world it is. But not the only one. Check out this World Map of Social Networks from Vincos Blog
As you can see, in other parts of the world like Brazil and China and India (very big places all), Facebook is secondary to other players.
Incidentally, I came across this map/blog via the blog The Daily Dish | By Andrew Sullivan, who had a link to this post World Map of Social Network Dominance on FlowingData that ended up linking to this blog.