TV spots are old-school, and don't get the return that they used to.
To really be a unique offering, the Witnesses have to have something special that can't be got anywhere else. They would have to get a lot sharper at locating those vulnerable and discerning people who are uniquely attracted to their message. My estimate in the developed world about 1 in 1,000 people are potential converts. To find these people requires cyber intelligence. That requires targetted advertising where these vulnerable people hang out.
The WTS is still doing random, scattershot approach and when they do get a nibble, they barely seem to know what to do with the fish!
http://sethgodin.typepad.com/seths_blog/2008/01/permission-mark.html
I think the campaigns and the carts are chiefly to keep the congregation busy, to give them a sense of purpose and duty.