Hmm, do we ever had any Open Source "answer" to anything succeed, not in the meaning of usefulness, but in the meaning of BEING ACTUALLY USED BY A HUGE NUMBER OF PEOPLE, something that is necessary for, say, Facebook?

Compare:

@ Linux, BSD -> Windows, OS X @ ogg -> mp3 @ Theora -> H264, @ Gimp -> Photoshop, @ Open Office -> MS Office, @ Mono -> Windows .NET CLR, @ GNUStep -> Cocoa,

The only exceptions I can think of are Firefox/Webkit (still lagging behind IE, though).

In the server side, of course, it's different: Apache, Bind, MySQL, PHP, etc...