1) Could you verify that the JFBCSystem plugin is enabled.
2) If it already is enabled, try changing the ordering of the plugin
3) Make sure there are no spaces between the braces and JFBCLogin - e.g. {JFBCLogin} instead of { JFBCLogin }
4) Check to see if you have a key set up in the JFBConnect backend > Social > Misc > Social Tag Admin Key setting. If so, you'll need to include it in the easy-tag, such as {JFBCLogin key=1234blah}
If all those suggestions don't help, please provide a specific URL and we'll try to help you further