Apple issued security updates for OS X Mavericks, Mountain Lion, and Lion, but not Snow Leopard (last OS X to run on PowerPC Macs). Gregg Keizer says one in five Macs vulnerable to attacks.
To Apple, Snow Leopard increasingly looks like Windows XP does to Microsoft: an operating system that refuses to roll over and die. At the end of January, 19% of all Macs were running Snow Leopard, slightly more, in fact, than ran its successor, Lion, which accounted for 16%, and almost as much as Mountain Lion, whose user share plummeted once Mavericks arrived, according to Web analytics firm Net Applications.
For my site, Snow Leopard users account for about 12-percent of all visitors.