This group is a effectively a trade association of individuals who work at the job of collecting taxes, i.e., tax collectors, both state and federal.
They've been reporting on this for decades, perhaps even a century.
This is the key takeaway from the report appears on the first real page:
Focus your attention on the upper right. State cigarette taxes have dropped about 4.5% in the last year or two; approximately $780 million US dollars.
Federal cigarette taxes dropped about 13.5% in the last year alone, a decline of about $2 billion US dollars.
What's I think more important is that it took about 65 years for cigarette taxes to reach this level.
At this rate of decline one imagines we'll be back to roughly mid 1990's levels of tax collection in less than five years.
Like you, I have "heard" that tobacco revenue for "government" is dropping.
This demonstrates the point quite clearly and unambiguously.
There are other pages of note discussing average prices of smokes, etc. but you really have to download and examine each year's worth of reports (I found 2011 and 2012, but not 2013).
From examining the site I would imagine 2015's report to come out later this year.
The only thing I can think of that corresponds to this drop is the rise of vaping.