OS X Mavericks' uptake slowed significantly in December, putting a crimp on Apple's plans to move customers to the new -- and free -- operating system. Data from Web metrics company Net Applications also showed a worrisome fragmentation of the Mac operating system market, with three older editions, including 2009's OS X Snow Leopard, crowded around the 20 percent line, each accounting for between one-sixth and one-fourth of the user base.