In general, it's hard to establish popularity of published works. The reason is that sales figures are the best indicators of popularity (in my opinion), and they are not public. (Unlike say, TV series or films whose ratings/sales are generally available).
To be fair, you make a good point about fan clubs and sites. But will an internet site's opinion actually represent general opinion? Most websites would actually have some niche audience, so their data may not always be reliable. Even if you took data to be reliable, what data would you consider a sign of popularity? I went on to goodreads, and tried to compare the popularity of two series, (The Farseer trilogy vs. The Wheel of Time). They had pretty similar rating values(about 4.1), and the wheel of time had about 2-3 times the number of ratings of the other series. (~20000 vs. 7000-10000). From sales figures, Wheel of Time is estimated to have sold about 40 million copies, with the Farseer trilogy estimated at below a million. So, though they show some correlation with number of ratings, it's hard to say what. John Ringo's books, which have together sold about 3 million, have about 250-750 ratings on average. So, even the number of ratings is meaningless.
What about fan sites? John Ringo does not seem to have a fan page on facebook, Robin Hobb has around 30000 likes, Robert Jordan about 65000, Brandon Sanderson about 20000. So even that does not a good indication of popularity.