Use the Amazon best sellers list. As of tonight, Canon has 13 of the top 20 best sellers. Nikon has 7.
Yes, that's where I normally visit for a rough gauge of interest level. And I usually take a peek once a week. Same goes for DPReview/Imaging Resource most-clicked cameras and BCNRanking.
All indicators show one CONSISTENT trend: dropping level of interest in Canon DSLRs. Point of reference? Just a couple of years ago, Canon DSLRs used to hog the various charts. Amazon bestselling DSLRs, BCNRanking, DPReview/Imaging Resource most clicked cameras etc etc. Even newly released cameras have great difficulties in dislodging the Canon DRebel or DRebel XT from the top of the most-clicked camera lists on DPReview/Imaging Resource.
It's true that Canon has not lost its lead in worldwide market shares for interchangeable cameras. But if the above trend persists and starts to grow in other parts of the world, Canon will surely lose their lead to Nikon. It's already happening in some parts of the world...
This is not exactly a bad thing. Canon will be forced to make some drastic improvements instead of plodding slowly along...
It was exactly a couple of years ago when Canon was paper releasing, or announcing the release of, the 5D III and 1D X. It is, therefor, not surprising that interest in Canon equipment began to peak around that time. Official releases came about a year later, which also coincides with peaking of interest in Canon equipment.
Other brands release on different schedules. Nikon seems to be releasing something all the time. Sony has had a few compelling releases lately. Therefor, it is again no surprise that interest in those brands is peaking now, or has sustained a moderately higher level of interest lately.
Canon has not had any major releases lately. The announcement of a white paint job on a preexisting camera doesn't count as a major "release", or even a minor one...it's non-interesting outside of the initial scope of the announcement. When Canon releases a 7D II, or the big mp camera, interest will once again peak, those cameras will be reviewed, and tested, and compared and discussed at great length. IQ and DR and megapixels will all become the heart of heated debates day in and day out...probably for the span of a year. After that, people will lose interest...and get back to more productive things...like making photographs with the 7D II they so hotly debated.
Brands of physical equipment exist in cyclical markets. They don't have the option of controversially eliminating discrete products and product releases in favor of perpetual improvement schemes for unreasonably priced, recurring, perpetual fees like Adobe and the rest of the software world. Every brand has it's ebb and flow, its peaks and luls, it's oscillations. Canon had their peak, and have entered a lul. They will have another peak in a year or two, and another, and another, ad inf.