I imagine it was useful simply because it was the biggest social site online.
If facebook had not existed I'm sure they would have found other ways to accomplish similar goals.
It's not as if they set facebook up with the intent on making it easy to organise revolutions.
If they could have got similar eyeballs by posting a video on xtube.com would that have made xtube a great innovation?
Why wasn't something else you say was actually present when it was needed?
Why didn't somebody else execute it and build it.
The fact is that <somebody> who can build awesome <anything> is difficult to find is the crux of this whole debate.