To bring in my own anecdotal audience, half of the people I know who watch the show stopped at various points during season two. Hell, until I got my new job late last year I thought that everybody else I knew who watched it had quit. So my personal experience lines up perfectly with Sci-fi's numbers.
The problem here is that, as you said, it's really anecdotal. And my experience, for example, is completely different. Everyone I've introduced it to has literally soaked up the show from start to finish (that's through S4.0). And none of them has complained about any significant drop in quality on the way. I should point out that most of these people watched it on DVD.
Again, this really doesn't mean anything since it's such a small number of people.
Also, it's interesting to look at DVD sales in addition to ratings. I have a hard time digging up numbers. Here are the only sales figures I found, from The Numbers
. I only found figures from the S2.5 DVD box set onwards. Also, all I found there were the total numbers for as long as the sets were within the top 30. Finally, these numbers are US only:
S2.5: Released September 19, 2006, entered the chart on #7, 3 weeks in the top 30, 263,516 units sold in that time, $8,997,580 earned in that time.
S3: Released March 18, 2008, entered the chart on #14, 4 weeks in the top 30, 303,458 units sold in that time, $11,821,313 earned in that time
Razor: Released December 4, 2007, entered the chart on #20, 1 week in the top 30, 147,768 units sold in that time, $2,953,882 earned in that time
S4.0: Released January 6, 2009, entered the chart on #6, 2 weeks in the top 30, 248,638 units sold in that time, $8,834,889 earned in that time
And here is a link (from the end of 2007, I must add) stating that nuBSG was doing very well in terms of DVD sales. I found some other sources saying it was doing well, but this was the only one where I was actually familiar with the source:
Battlestar Goes Boldy Into The DVD Universe
(New York Times, Television)