Looks like different version of the plugin generates files in different places.

asfernandes · asfernandes · commit c65bb711d8e7 · 2016-05-30T16:24:56.000-03:00
diff --git a/src/etc/bin/copy-javadocs.sh b/src/etc/bin/copy-javadocs.sh
@@ -10,4 +10,4 @@ fi
 BASE_DIR=$THIS_DIR/../../..
 
 rm -rf $BASE_DIR/apidocs
-cp -rf $BASE_DIR/src/fbjava/target/apidocs $BASE_DIR
+cp -rf $BASE_DIR/src/fbjava/target/site/apidocs $BASE_DIR
