Apple warned in a recent Support Note that after June 30, Macs running a number of older versions of OS X won't be able to use their mac.com and me.com addresses as AOL Instant Messaging (AIM) IDs to log into the AIM service with iChat. The company said users must upgrade to at least OS X Lion 10.7.2.
To ensure a secure and reliable experience, customers using a mac.com or me.com ID to log in to the AIM service via iChat must upgrade their Mac to OS X version 10.7.2 or later by June 30, 2014 to continue using the AIM service.
Certainly, most Lion upgraders will move up to OS X 10.7.5 or to Mountain Lion 10.8.x, the latter of which offers much improved performance.
If users can't upgrade, then they will need to create a new AIM ID from the AOL site. This will require users to rebuild their Buddy lists from scratch and remove the old ID from the iChat preferences.
Apple's success with the transition to OS X Mavericks in the Mac community might make some wonder about the magnitude of the issue — according to Net Applications' Net Market Share counter, Mavericks has a 51-percent share. However, the share covered by this Support Note adds up to 19 percent.
There's an interesting history of AIM at Mashable by Jason Abbruzzese. There's a Mac part of the story. Ages ago, the first iteration of iChat was powered by AIM. iChat supports iCloud, MobileMe, AIM (AOL Instant Messenger), Google Talk, Yahoo and Jabber services. In Mountain Lion, iChat was superseded by Apple Messages.