Obviously the code you inserted works here
Ensure that you have the latest version of the jfbcsocial plugin installed and enabled. We just updated this (along with all the other extensions) to 3.1. I'm pretty sure the 3.1 social plugin will work fine with the 3.0.x version without any issue though, if you're not ready to upgrade to 3.1. There shouldn't be too many problems if you decide too, but we understand that not everyone wants to upgrade all the time.
The upgrade and installation guide is linked above.
Hope this helps, and good luck!