Before Apple released OS X 10.8 Mountain Lion, they released a preview of their Messages app for OS X 10.7 Lion users. The app has since evolved and been released with Mountain Lion and it doesn't look like Apple is willing to let OS X Lion users enjoy the Messages app for much longer.Since the final version of Messages will not be released for OS X Lion, the only way to continue to use the software is by upgrading to OS X Mountain Lion. Apple is currently sending out notifications to those using the beta that the app will stop working on December 14, 2012.
Upgrading to OS X Mountain Lion will cost no more than $20, which doesn’t seem like much considering the advantages of using the Messages app. Messages for OS X allows you to stay in touch with your friends and family who use the app on their Macs and iOS devices.
However it is important to remind our readers that it may not end with a $20 expense. Upgrading to OS X Mountain Lion may also require users to upgrade their Mac hardware which we all know can be quite costly.
source – CultOfMac