Without published USA accounts , we can only surmise somewhat. The only tangible evidence that I have seen is in the UK accounts which show fairly much a flat-line of 20 million UKP income for the last 7 years... http://www.charitycommission.gov.uk/registeredcharities/showcharity.asp?remchar=&chyno=1077961
Canadian and Australian figures are broadly similar.
A study going further back indicated that there was approx a 25% fall-off when the "paid for" literature arrangement stopped , though this did recover somewhat.
Anecdotally , my understanding is that the income in the USA is broadly the same over the same period (ie no increase but no decrease in overall contributions). While direct donations have gone down slightly , this has been made up by legacies ,as a result of increasing equity and property values. In the UK in 2006 , total income was approx 7.6 million UKP donations and 3.8 million UKP legacies.
The WTS really needs to increase income or cut costs by about 5% per annum. In the absence of the former , the WTS have achieved the latter. Their main costs are newsprint , which hasn't really moved for the last ten years , and fuel which has increased considerably. The recent cost cutting measures (Bethel lay-offs , selling of buildings , WT & Awake on a monthly basis etc) have reduced their cost base considerably.
Having done this , the main issue that the WTS needs to address is the vast amount of literature virtually "given away" in the 3rd world. I'm advised that is the main reason behind the plethora of "special tract campaigns".