It comes up all the time, and there’s no perfect answer to the question, however I would say with pretty reasonable confidence that the market share in mirrorless in 2015 went like this:
Putting actual numbers on that is a bit tougher. We know that Olympus shipped 550k units in their last fiscal year (though that will slip to 460k in the current year according to their estimates). We had Sony at about 12% of the overall ILC market last year, which put their mirrorless number somewhere around 1.4m. And from Canon’s recently stated numbers, we’d have to guess that they were somewhere above 300k units for mirrorless last year, and are now growing significantly.
It’s tough to assess the precise positions in mid-year—especially with this year’s quake affecting most makers, but not Canon—but I’d say those three positions are still pretty much a safe bet at the moment, though the order is changing. Canon will almost certainly pass Olympus this year and move into the number two spot. Olympus isn’t seeing sales of the current lineup improve, and seems likely to only have the new EM-1II in short supply after launch, and at their usual high-at-announcement pricing, which means it won’t move the Olympus numbers positively. Meanwhile, Canon’s numbers have been going up, plus it looks like we have two new EOS M models about to pop.
The real “loser” at the moment is Nikon. Overall in ILC (DSLR plus mirrorless), Canon is a clear #1 with mid-40% market share, Nikon is #2 with mid-to-high 20% market share, and Sony third with a bit less than half of Nikon’s market share. Overall, these three easily take over 80% of all interchangeable lens camera sales, and perhaps as much as 85% (model introductions and things like the quake are moving numbers a bit more significantly this year than previous years).
But Nikon might not even be #4 in mirrorless, and that’s got to be troublesome, as mirrorless shipments are staying relatively flat while DSLR shipments—where Nikon is #2—are declining.
Of course none of this speaks to which camera you should buy. Not a single one of the Japanese camera makers is likely to step away from selling cameras, despite the collapse of the market. So don’t read anything into this article other than the fact that two brands—Sony and Canon—have healthy mirrorless businesses at the moment, and are more than likely to dominate the news cycles in the near future.