Neighbors not using it shouldn't mean much, as most flies never get more than 1/8 mile from where they were hatched. 

Did you ever figure consumption?  Were you mixing salt with it?  That is the problem.  To be effective, consumption must be steady and regular, and most guys trying to monitor concumption with dry mineral will go bonkers in short order.  The consumption issue would be the same with blocks, too...then there is the weathering issue.

You DID help things by having it out and available to them, even if you really didn't "see" any results.