Blogs vs Books
Writing books is an alternative way of spreading ideas. If we assume that, on average, each copy sold represents two hours of time reading--allowing for some buyers who stop at the first page, others who spend much more than two hours reading and perhaps rereading--that corresponds to sales of about five thousand copies. A book that sells a total of five thousand copies isn't a flop, but it isn't doing terribly well. While I don't have figures readily available, my guess is that all of my nonfiction books, with the probable exception of my one textbook (Price Theory), have done substantially better than that.
On the other hand, time I spend making blog posts mostly is not time I would be spending writing books instead.