Well it's an accumulation of indicators that point toward CivBE player base being approximately 1/10 - 1/8 of Civ5. Steam numbers and numbers of forums thread and viewers point toward this.
But that doesn't mean much in terms of whether or not the game is good (popularity is never a good indicator of quality). Just that it's not very surprising to have less threads about civBE as a result.
The competitive scene is totally empty for CivBE though (MP, no Gotm, no HoF, no "Apollo challenges") and I'd argue that has a direct impact on the number of threads on CFC. Hard to discuss strategies when very few people even care about them.