Erro: Unable to merge dex when add cordova-plugin-facebook4

There is a long running thread about the Facebook plugin, e.g. https://github.com/jeduan/cordova-plugin-facebook4/issues/507. The gist of it is library incompatibilities and many people including myself have used the 1.7.4 version of the plugin to get it to work. I.e. you need to remove the version of the plugin you have and do this instead:

cordova plugin add cordova-plugin-facebook4@1.7.4 --save --variable APP_ID="<myid>" --variable APP_NAME="<myname>"