I think all you have to do is look at the numbers of other Christian denominational conversions, and compare it to the Watchtower organization's conversions to see how paltry the WT numbers really are. As of now, it takes up to a couple of thousand man-hours to produce one convert. In the United States, where the JW religion had it's birth over a century ago, one would certainly expect more than a million members. When you factor in the fact that most baptisms in the U.S. are JWs children, this further illustrates the sorry results of their "preaching".
In the first-century, people were accepting Jesus Christ in astounding numbers. What is so impressive about this is that the preaching activity was a spontaneous manifestation based upon love instead of any centralized organization pushing people into the field. Also, these Christians were not motivated by the "salvation by works" motto of the Watchtower. There was also no regimented and organized work that was micro-managed by a centralized corporate structure.
There are other denominations that are younger than the Watchtower that have produced amazing results, and have more members than the JWs. If one wishes to attribute divine favor based upon the number of members, then the Catholic church must be hopping with holy spirit.